First thing I would do is replace the relay that's in the left rear corner of the engine bat, between the vacuum canister and the cruise servo. Should be under a plastic bracket, but could very well have fallen off and is now just resting on the inner fender. In a '88 model there should only be one relay there, it is the one for the dually/tow lights. For 89 and up there can be two of them if the truck has the E4OD transmission, one is again the dually/tow lights relay and the other would be power supply for the E4OD - there really ain't no telling which is which without looking up the wire colors in an EVTM, which is actually an item I highly recommend every IDI owner procures for their truck. Without an EVTM (or help from someone with one) you're more or less flying blind when it comes to things that are less obvious than ignition switch power supply and such, factory EVTMs are still available on ebay for reasonable cost - ours was like $20, yes it was used but so what, it was and still is in very decent shape and perfectly usable.
Nah, you're right, missed the cab roof lights part. But, we've had that issue as well - in our case someone had creatively rewired the rear ICC lights (the three under the tailgate), don't recall what exactly they had done but the result was once the relay was energized by the switch it would power its own pull coil via the ICC lights back-feeding into the main markers circuit off the switch, resulted in mostly normally bright fender lights and dim everything else. An improperly wired trailer connector can do the same as our ICC lights, especially if it's tied into the bed rear lamps via a breakout harness.
