A few decent questions-the burn victim.

Ok Tach sounds like someone shoved a 4cyl tach in there, if not, it needs to be replaced regardless.

9 times out of 10 hunting idle is exactly what you guessed, Vac leak, dirty or bad IAC.

Trunk release could be simply un-plugged. So owners don't like the fact that it is always HOT, and if your windows are down, someone can reach in and press the button to open the trunk without needing the key.

Rear Brakes, they are a pain in the A$$ plan on replacing the rear calipers, or take it to a shop and let them deal with it.  Unless you have a proper caliper turn in tool and patience.  Cheap 4 sided tool is useless.

Fire wall plugs are actually easy if you have long arms.  You can't see them, but you can feel for them and it gets easier over the years once you are use to it.

If you bring the car to the Tech Session the scan tool is available to members to use for a one time $5 fee.

        Head units I don't know the exact year change up, buy I know a lot of people have used the Trans Am GM CD Player as it lights up Orange like the stock Fiero deck.   I have a 99 Chevy Venture Van CD / Tape Deck combo that I'll be putting in my 88 some day.  Just sitting here in a box.  It does use different plugs and I was just going to go to the wrecker grab the stock plugs, then go find an Fiero Adapter harness and mod the 2 together.   That or just say to hell with the Stereo all together and go fully digital.  The wifes new Cell Phone has a nice digital FM Tuner and 4gb on-board music storage, all touch screen interface.  All in something the size of a credit card.  Kinda makes stereos almost seem useless.   (Yes I know I would have to then install amps into all the speaker channels in the car.

Now as to the rear brakes, lets see I have tried 3 seats of rear brakes, and 2 out of 3 ended up with the piston seal going south and leaking.  It primary cause is the 84-87 E-Brake systems are horrid.  Which leads to cables going bad and then people stop using the E-Brake.  if the E-Brake is not used, the mechanism inside the caliper that turns in and out sizes up.  You have to rotate the piston IN as you compress it, or remove the E-Brake arm from the caliper and hope the mechanism rotates as you press the piston in.  Unfortunately, it rarely ever works.   I won't even take the job on as a paid job.  Shops have a much better caliper turn in tool that actually loads the piston as it is turned in.     All of our personal 84-87 Fiero's have the rear grand am brake upgrade, which deletes the parking brake and hence no more problems.  My 88 does have a working parking brake, and it is to this day the Only Fiero that I was able to save the rear calipers.

Hit up these guys.    http://captfiero.com/ppc/   Sounds like you got a real doof at the parts counter.    I run the MSD Superconductors on my 88GT V6, My 857 GT V8, my wifes and my mothers Fiero's both have the MSD Premium Wires.   I know of at least 8-10 others that run the Superconductor wires, there a really good wire, the only drawback if you have to make the wires yourself.  They come in a kit that you have to cut, strip, attach and build them by hand.  

       As for the coil is should be around 50 bucks.  

If you want to get "raspy" sound, simply remove the cat and replace it with a straight pipe.  

                    The ADS Chip is actually costing you power.  They are horrid, the only chip that works well on a Fiero is a custom chip designed for your engine.   The only thing the ADS chips did was increase the timing, removed the Rev Limiter, and in some cases removed the EGR system.   If your car is stock, the stock chip is the best bet.       If you do continue to run the ADS chip, run 89oct gas as the minimum or retard your timing a bit.