91 200 with BOSE system and aftermarket stereo unit
 

I recently purchased this car and previous made Car Toys install aftermarket head unit,
but they didn't by pass BOSE speaker amps and didn't use pre-amp output on HU,
so signal go double amplified.
As result rear amps failed, fronts are still running.

Question: can I connect original 2 Ohm BOSE speaker without amp to aftermarket head unit designed for 4 Ohm speaker?
Is it safe for head unit?

Yes you can. be sure to use ground from HU to speakers. Do not use the common ground. Did you confirm that the rear output from the HU is still working? I would do this first before cutting or rewiring anything. If HU is dead I'd be back bitching to the install place.

The manual for HU says do not connect 1-3 Ohm speakers to it, only 4-8 Ohm...
I also did some more research and few people complained that such setup fried built-in amplifiers in the HU.
HU is working fine, but BOSE speakers have high pitch noise....

Hey, I don't post here much, but had to chime in on this. When I got my 5-speed, the previous owner had put an Alpine head unit in, and connected it to the stock wires, frying all the amps. The Bose system sends a "smaller" signal thru smaller wires to the amps which boost it right at the speakers theoreticaly resulting in less "noise". The system sounds great when perfect, and 20 years ago, it was certainly state of the art. The monkey's at Car Toys should have know that, and since it's about $50-$75 per amp to fix....sounds like they might owe you $200 since the amps/speakers WERE sellable. If it were me, I'd work them for some new speakers, as you may not want them tearing your doors apart. The 20 Y.O. dried out paper cone Bose speakers you have will NEVER sound anything like a newer set of proper speakers with a different head unit. I bought new Alpine speakers and ran new wire to the rears. The front's are actually 4-1/2", so had to rig up a way for the 4" to fit in the stock Bose box, but was going on a roadtrip in the morning, and used the thin wire that was there. The front's sound thin and weak, and have been meaning to run some real wire(1.5 years), but the rears sound soooo great, it's on the back burner. The stock Bose system in my other V8 is rebuilt, and sounds great, but lacks the desireable IPOD jack/XM radio.....New speakers and speaker wire, and you'll love it.....Tom