I did my own stereo in my 91 350 SDL a couple of years ago and the only amps I found were under the back seat's back rest. To get at them you have to pull the back seat and then the back rest. Up front I just pulled the speaker wires and reconnected them to the new head unit. One thing to be aware of though is if your car is like mine there are speakers in the doors so you want to make sure you hook those speakers up as well.

Good Luck.

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91 350 SDL - 155k and still purring
95 SL 500 - 88k Wife thinks its hers
05 F350 XLT FX4 4x4 Super Duty Crew Cab - Yea Baby