here's a condensed version:
GM didn't do it but YOU can.
Requires no soldering iron. In 30 mins all four taillight bulbs will light up with brakes/turns.
This were done on a '92, 88-89's have different bulbs (circular base) than '90-98 trucks so same procedure, but will need a test light to find proper contacts.
Remove taillights, unplug 'em, remove circuit boards. Use 5" small strand of wire, strip ends to 1cm of bare wire per end. Below are instructions for INDIVIDUAL CIRCUIT BOARDS.
DRIVER SIDE:
Remove both running bulbs (can remove reverse bulb if wanted, but it won't be modified), diagram's below.
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It is pretty straightforward what's needed. For middle bulb (between top, reverse bulbs) place wire (or thread it through "contact rod") to stay in place as bulb is installed. Bulb can be facing either direction, as long as bare wire end is in correct place. Install bulb with bare end of wire touching/threaded through BOTTOM-RIGHT contact.
Top bulb is written "BRIDGE" for ground contacts. Circuit board does not have ground for "High filament", so take a SMALL strand of bare wire (1cm at most, don't want to touch another contact) thread through "contact rods" on ground end of bulb by wrapping it around base. This is NECESSARY. Carefully install "High filament" wire in designated spot, install bulb.
With both bulbs installed, plug in circuit board, hit brakes to test. Both bulbs should light. If nothing happens, 15 amp fuse was popped. If only middle bulb lights, something's wired wrong, or wire/ground slipped off contacts. Re-apply and try again. Here's how tails should look:
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PASSENGER SIDE:
Same as above, but contact points are a bit different. Pay CLOSE attention to diagram below:
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When you're done, it should look like this:
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Here are both bulbs lit up for brakes on driver side.
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That's it.