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Shown below is a Sony head unit I had already swapped in but was using with the stock amplifier and speakers. Connected when I swapped in the head unit was PAC SWI-X steering wheel control adapter. It all worked very well, in fact the Sony lets me turn the radio on from the steering wheel controls unlike the stock radio. I found that the head unit sounded okay at low to medium volumes but the stock speakers sound like crap if you try to play any real bass. The stock Monsoon system has an amplifier built into the passenger rear quarter panel area. The Monsoon amp uses the radio speaker outputs as its line level input, no RCA pre-outs. The stock radio uses one wire in the harness to sense the presence of the amplifier which auto powers on with no signal from the radio like normal amp turn on signals, it seems to sense speaker signals coming from the radio. I believe the stock radio then turns its own amplifier gain down so that you can still use most of the volume knob range. With the Sony in place the Monsoon amp is really loud with barely any volume on the Sony deck. It worked fine but I could only use maybe 40-50% of the decks volume range before blowing the speakers. However, the Monsoon amp also becomes really shrill as you turn it up, as you do, the bass notes start sounding like someone is hitting a piece of bristol board. This is the reason for putting my own amp and speakers in. I installed a Pioneer 2 channel amp to power the front speakers which I changed to MB Quart 6.5" Coaxial (DKF116). The rears are now Polk DB 650s and are powered by the Sony head unit.
The amplifier connector is the one place in the car where all the speaker wires come together. Since my car is a convertible. There are 4 full bandwidth speaker outputs (8 wires) entering the amplifier. The amplifier outputs separate teeter and woofer for each of the 4 speaker locations (16 wires, 4 per speaker). I just bought a radio adapter for a GM radio that happened to match the amplifier plug. I pulled out the existing pins and plugged them all in in the pins I wanted. Just note the wire colours at the radio and at each speaker. In the picture below you see four wire loops. These simply loop the rear speaker feed coming from the radio back out to the rear speakers, in this case the tweeters only. I connected the Polks to the stock speaker's tweeter feed wires. The other 4 wires lead to the 4 spade terminals shown. These take the new amplifier outputs and connect them to the front speaker tweeters. The MB Quarts are also connected to the stock speaker's tweeter feed wires.
Leading to the right is the factory harness. Heading left is the new harness prior to wrapping it up.
The 2 black harnesses are the adapter harness in one and the RCA pre-outs in the other.
Here is the amp temporarily placed in the spare tire. The spare is flipped around compared to the stock orientation. Later I mounted the amp on a 1/4" piece on Masonite. The spare is tie wrapped into place since there is no room for the normal wing nut and bolt. Then the amp on its board is tie wrapped into the spare. This lets the spare be retained and the amp occupies previously wasted space.
Here is the amp pulled out on its harnesses. I left the wiring nice and long so that the amp could be pulled out the way in the event I need to use the spare tire.