Speakers - question
 

I have a 2003 SR5 without the JBL system.

I decided to replace the rear speakers as I kind of like the front speakers but think the back speakers sound pretty bad.

Anyhow, I replaced them with Alpine SPS-170A's which were not an exact fit to the factory hole (there is a small amount of room around the bottom edge). The Alpines actually sound worse than the factory speakers. The Alpines might be a bit better on higher notes, but they sound a lot quieter and have less bass.

Is this because of the small gap around the speaker or is this a lack of power?

My next plan was to make an exact fit bracket out of MDF so that the speaker is sealed better. Is this the right thing to do?

Dont even try to put some mdf wood thing. The alpine speaker is not feeding enough power to sound right. To get out the full potential you need to know how many watts (rms) that alpine can handle and get yourself external amp.

Building rings and or adaptor plates should help out with the overall sound. It also sound like you may have the rears wired out of phase from either the fronts and or each other lt/rt. If this is the case it will dramatically decrease the lower end bass reproduction. Take and flip the +/- on one of the rears and see if you get some of your low end back.

speakers
 

I had already tried reversing the polarity in case I had wired it wrong, but that did not seem to make a difference.

I wired black to + and yellow to -. (I only tried the left side, and put the factory speaker back in when it did not sound good).

You probably have a lack of power to the speakers, factory headunits don't provide enough watts to really use an aftermarket speaker, hell aftermarket headunits barely provide enough power to run a set of speakers. Get a 'line out converter' and a 25x2watt amp, it will sound much, much better.

either buying a ring or making a ring will help with sound quality alot especially with the bass. Second i agree with los gates you need an amp for them. the oem head unit might push 10 watts rms if you lucky, the speaker is not being pushed very well. so the lack of power is effecting its volume not its sound quality.