Tail light fuse blows 94 Chevy S-10

About 2 weeks ago on a rainy day, the fuse for the tail lights and internal panel illumination blew out on my 94 Chevy s-10. I replaced
the fuse and it blew out again during driving the car at low speed, but on a semi-bumpy road. The next fuse blew immediately upon start of the car. The fourth fuse blew when hitting a pothole (it seemed). I put the fifth fuse in tonight and drove around the block and it hasn't blown yet.
Here are some facts: I have no illumination lamps for the license plate, but cannot remember if they EVER lit up at all anyway. I recently had a new catalytic converter welded onto the tail pipe assembly. There has never been any type of custom modification of the electrical system. When the fuse blows it causes the interior CAB light (the one in the roof over the head rests) to come on. Almost as long as I've had the car, both of the "banana" switches on the side dash/door panels have bad connections; jiggling them will cause the courtesy lights (foot well lights and the cab light) to go on and off (either the right or the left side switches seem to be bad in this case). No other fuses seem to have blown. Blinkers, hazard, brake and park lights work in all circumstances. The problem seems to be intermittent with only one fuse blowing when starting the car.
I have removed the tail light assemblys and reseated all the bulbs in their sockets. I can see no signs of wear to the cabling, although the tape on the wires is crispy.
Note: in regard to the work on the catalytic... I watched the guy perform the welding and removal of the old part and he never opened the hood or anything, is there any way he could have intentionally or otherwise sabotaged the wiring or "grounding" with his work?
Any ideas on the situation are greatly appreciated. I know it's almost impossible to diagnose, but I think the internal DOME/CAB light that comes on is quite odd when the fuse blows.
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On Wed, 25 Feb 2009 02:39:24 GMT, .@ wrote:

Most likely scenario is that one of the wires in that circuit has been rubbing on something, has worn through, and is now grounding out intermittently. Back in the day the first place to look was the wire supplying the license plate light. FWIW YMMV
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On Sun, 01 Mar 2009 12:55:33 -0500, NickySantoro

The fuse keeps blowing... everytime I drive the goddamn thing a Mexican garage keeps charging me more money... now I'm up to thousands of dollars...
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It should not be that hard to visually inspect harness from rear light and rear of truck to cab. I have little doubt that a "proper" inspection of it will yield cause of problem.
On 3/5/2009 11:29 PM, .@ wrote:

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Would you keep paying someone who did not fix the problem in the first place?

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