I have this option and very little use for it.
Engine doesn't get warm enough when you start the car and windows have frost or ice on them. That's about only time I use it because scraping ice on rear glass is a pain w/ most scrapers. Start the car, push switch then clear other glass.
Side Note: Make Sure you have a spare door key doing this because very easy to lock yourself out w/ engine running. Many people automatically hit the lock getting out of car w/o thinking even w/ manual "Fiero" style interior locks. Easier still w/ any power locks even before near everyone has fobs. (Many newer cars try to fight you locking w/ engine running and driver gets out.)
Note that the Rear Defog only works for a few minutes even when you push the switch.
when push First time stays on 10 or 5 minute. After until turn off I-key 5 or 2.5 minutes.
Because the "Relay" is a Timer Relay and GM used 2 or more timing setups over the years. Don't count on GM and other doc's always saying 10/5 or whatever because doc's don't get updated for timing. The Timer Relay use here have timing data printed on them. My 87 has 5/2.5 timing while older models may have 10/5 timing.
I believe RD wiring is only on cars came w/ the option. Isn't like Power Latch wiring for deck lid that mostly complete in many to all Fiero covered in my Cave,
Trunk ReleaseNot digging thru my parts car w/o this covered under cover and stuff. Don't remember seeing a "spare" plug in the dash for this. You see 2 plugs under blank cover or power lid cover if has any wiring or when you pull back dash to change dash bulbs.
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)
The Ogre's Fiero Cave