Has anyone fairly easily added an engine compartment light, to a non-light car, without replacing the whole rear lamp harness?

Thanks for the help.

It's really quite simple. All you need to do is add a small in-line fuse (5A is enough) from the battery junction block under the passenger side engine grill, then run a wire from the fuse to the lamp location. For the lamp, do a quick search here on PFF and you should find the thread where someone found an identical trunk lamp housing as the Fiero's, except that it has a small switch incorporated into it as well. The lamp housing grounds against the trunk metal if I remember correctly so one wire is all you need.

You'll have to remember to turn off the light though. If you want one that turns on and off automatically, then you can run the power wire to a pin switch first, like GM did for the '84 and '85 models. Mount the pin switch to the small metal tab where the trunk wall meets the driver's side strut tower. Adjust the height until the underside of the decklid contacts it when it's closed. If you're not sure what I mean, I can post a picture of where the earlier cars had the switch.

Look for a C4 Corvette under hood light. They have a mercury switch that turns the light on and off when hood is raised and lowered.