To the electrically minded...

You just need a normal 4 pin relay to do this, and take the feed from the reverse light as the switch. I did this in the battery compartment.

-ve

+ve In to relay +ve out to light on back cut into the feed to the reverse light switch which screws into the gearbox just under the middle seat so if you pull it out its easy to get to and put this on the switch pin.

This way, when the reverse light comes on, it will switch the relay and turn on the new one at the same time.

After a bit of rumaging...

87 to permanent +ve feed 85 to ground 86 into existing reverse light wire 30 to new light.

What switching do you want?

'and' or 'or'

Mark has given the solution for 'and' - if I read it right.

To make 'or' you could use a pair of diodes on the +ve feed wires and connect both to 85 or 86 (some relay coils are polarity sensitive)

You've lost me on this one, why do you want/need a relay? Do you not want to just turn it off and on? A plain vanilla switch?

For example: 60w lamp at 12v draws 5amps - meaning you need a switch (and wiring) at the dash that can handle that load. Using a relay means the control circuit can use much lower rated components.

Apparently not.

And why the connection to the reversing lamp, is it some kind of legal requirement?

Ive got mine wired up like that so that the extra one comes on when i put it in reverse with no extra switching.

I'd do that in the pinz but it ain't got no reversing light! I've caught a few people out like that, it seems the 2.5 tonne 6-wheeler isn't actually reversing until a little white light comes on. A few stubborn horn honks from them followed by the relentless movement of the truck soon sorts out right-of-way issues.

Why put a switch on the dash, put it in the lamp!

Ahhh.. my lane-change theory in the truck in action (from a previous life driving them for a living).

44 tonne+indicators=right of way. Any audible toot of horn during said manouveur may cast doubt upon overall public safety, but the HSE says the other party should take evasive action ;-)

Last time it happened to me was a heirarchical bully situation, I was tooting through a small town, came upon a section of road that had yellow hatching and only room for one vehicle, the way was clear so off I went, only to meet an 18-wheeler coming the other way using the "bigger than you" rule. Behind me was a Ferrari 360 who wasn't at all happy that I was reversing at him so he sat there honking his horn so I stopped, made sure I wasn't actually on him, then carried on reversing, he got the message after a while.

Another one was some oafette in a car park who was tailgating me, so when I had to shunt backwards to get into a space she was right on my tail, honked her horn and gave the international chav sign for "you dissin' me" (hand outstretched, palm upwards, pig-like expression with mouth agape) and lent on her horn, briefly, but had to give way in the end.

One I saw in a car park made me want to break out the ratchet handle and bash some sense in. Van driver reversing out of a space, dolt comes round the corner, sees van reversing quite some distance away, accelerates up behind van and honks horn. Aargh!

I think it's illegal in most places, hence me thinking there was some legal reason for the OP's request to wire the lamp into the reversing light circuit, so it could only come on when the gearbox was in reverse, I thought perhaps it was a requirement, as I'm thinking of doing the same thing I thought I'd barge in and ask. As it turns out I think he only wanted to do it so it would come on as an extra reversing light.

Thanks for all the replys.

Worklight in this instance is a light that when the disco is being loaded up is iluminating the rear area - not to be illuminated whilst driving. The thought was then that as it is on the back, and its white, and the dimwits that park near where the vehicle is used (central london) then to have an extra reverse light and also to light up where you are reversing would be a good idea.

So in effect what i'm after is an OR

Either - light switched on by switch in rear load bay OR light switched on by engaging reverse

How does that stack up with the wiring suggestions (being tad thick - just back from a presentation lunch thingie burp......)

Si

You have choices .....

Option: 1 Relay + two diodes control (reverse) >> diode >> 85 control (courtesy) >> diode >> 85 ground >> 86

+ve feed >> 30 load >> 87

Option: 2 Relays #1 control (reverse) >> 85 ground >> 86

+ve feed >> 30 load >> 87 #2 control (courtesy) >> 85 ground >> 86 +ve feed >> 30 load >> 87

In both cases don't forget to wire the other side of the load to ground.

The above should be considered an OTTOMH doodle for discussion purposes only. Note: 85/86 are polarity sensitive on some relays and may need to be reversed in the above.

All those sill numbers confused me but a google and I think I have them sussed.

For the switch find a suitable +v through switch to one side of relay coil (86). Take feed from reversing light to the same point. If you don't want the real reversing light to come on with the switch bung a chunky diode in this lead (end with band to the relay). Connect other side of coil to chassis (85). Feed (87 or possibly 30) with +v suitable for the lamp load, connect (87a) to the lamp and the other side of the lamp to chassis.

I think that you will need to be careful what you do if road use and MOTs are being considered. Apart from what has been suggested earlier there are wattage and approval mark requirements once you get into those territories.

Specifically start with Schedule 14:

14: I would imagine that providing it could be isolated for said test........