Rear Right (+) Blue/black
Rear Right (-) Brown/orange
Rear Left (+) Yellow/black
Rear Left (-) Brown/orange
According to the above information. If those colors don't match, just take a look at the speaker connection. The positve plug will be bigger.
And no, it is not alright. You must tap both left and right for propper signal. Tap them at the factory amp though, and it will make it much easier.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>