I noticed that the PO of my 89 vert had an aftermarket head unit installed without hooking up under dash speakers. Doors and headrests working as front/ rear. To hookup under dash as well, Would I hook up parallel to L/R door speakers, or would that be a strain on the alpine head unit?

 

It's usually not recommended to do that because each speaker output is designed to work with a specific impedance. You probably won't break anything, but the volume won't be as loud.

But if you have door speakers then the under dash are irrelevant. If you have decent doors, then they should provide all the mid range you need (and most of the high range).

If you intend to use the under dash to augment the high end, again you will get poor performance as they are pointed at your knees.

If you want more highs get tweeters for the dash or doors, if you want more lows get subs. But I don't think you need under dash if you already have doors.

 

That's kinda what I thought but was a little confused why PO put new Pyle 4" speakers in the dash and didn't hook them up. It's got 5 1/4 alpines in the doors, I added a sub to the spare tire well. Might put some good 6" in the doors and call it good.