I think you're okay as you are -- 150 watts at 12 V is 12 A.
Seldom will the car be all the way up to its alternator capacity of 55
A. It's worth thinking about because in the winter you're more often
running a few to several power-hungry accessories at once (headlights,
defroster or heater with its blower motor, wipers...) and perhaps are
more likely to be sitting in traffic at idle, where your alternator is
not making its full output, though it's still better than the
old-fashioned generator under those circumstances.
When this happens the car is essentially running from the battery.
Ammeters tell this tale directly by sagging a bit to the negative side,
though of course you can infer it from voltmeters. (It has to be
pretty gloomy to trigger an idiot light.) When you pick up a little
speed the ammeter swings to the positive side for a while as the
alternator pays back the battery.
Anyway, if you're not scaring its full capacity I wouldn't worry. What
you don't want to do is go into the winter with extra loads and a dodgy
battery, so make sure it's in good shape and has nice clean solid
As for overloading switches or whatnot... operate them with an
aftermarket relay, and a *fused* line from the battery to said relay.
Parts and kits are pretty readily available these days because of the
popularity of both auxiliary lights and large trunk-mounted stereo
PS. You didn't mention whether you are configuring these as proper
DRLs or manually controlling them with a separate switch. I assume
that by "auxiliary driving lights" you mean the ones that are like
extra high beams, rather than fog lights. Especially in the former
case, be sure to turn them off at night when there is oncoming traffic.
And don't make the opposite mistake either, of failing to turn on
your "real" headlights (and thus taillights too) in dark or rain, just
because you have a DRL-like accessory. Not that YOU'D make either of
those mistakes, of course, but since so many drivers do, and one never
knows who's going to read these postings, I thought I'd mention it.