This should solve your problem. They have a 2 channel and a 4 channel. Just hook up the amp switching lead to a switched power source.
Blaupunkt Car Stereo Amps Solve Problem of Missing Aux Inputs
Blaupunkt PCA-465 amplifier.
Broadview, IL (6/19/06) -- Blaupunkt has introduced a line of car audio power amplifiers that offers separate audio input connections for iPods, portable MP3 players, or portable navigation systems. Since most factory car radios do not have auxiliary inputs for such devices, the Direct Aux Input® feature of Blaupunkt's ProComponent PCA series amplifiers provides an external player connection and significantly boosts the power of the car audio system at the same time.
PCA 465 auxiliary input connection diagram, shown with optional MP3 player
"While all car audio amplifiers are designed to deliver more power to the loudspeakers," said George R. Velez, Marketing Manager, Blaupunkt USA, "the these PCA amplifiers also provide a simple but effective way to support portable music players and navigation systems without changing radios or compromising performance by using an FM adaptor."
The Blaupunkt ProComponent PCA line consists of three models, the PCA265, PCA465, and PCA1350 rated at 2x65 Watts, 4x65 Watts, and 1x300 watts respectively. All three have conventional line-level RCA inputs and high-level inputs that can driven by the radio's speaker outputs. The multichannel PCA265 and PCA465 also add stereo Direct Aux Input® jacks to allow connections to portable media.
The reinforced Class AB circuitry is stable into 2-ohm low-impedance loads, and the multichannel amps can be bridged to achieve higher output levels. The crossover filters, both high- and low-pass, are continuously variable from 50 to 250 Hz.
All the Blaupunkt Pro Component PCA amplifiers are currently available. The PCA1350 has a suggested retail price of $249.95; PCA265, $159.95; and PCA465, $239.95.