I got a new car earlier in the year (well, new to me!). It has a very nice radio in it, and the speakers aren't bad. It has DAB, USB , Bluetooth, and, after a long while looking for the slot, no CD! It also lacks a line output or remote line to switch on my after market sub-woofer and amplifier, but it does have an unused speaker output, so I knocked up this:
Now the issue is, the output from most head units is bridge-tied load, to get a bit more power out. This leaves the speaker lines floating at half supply (6V) to chassis. Never ground a speaker wire!
This is OK though, as we can use this to trigger our remote line.
One side of our speaker is connected via R10, to the gate of a small FET (Q1), this, in turn switches on Q2, which supplies 12V to our remote line, to trigger our amplifier on.
IC1A gives us a low-current half-rail power supply, we can use to reference our audio to, as we can only "swing" our audio above the chassis 0V, rather than swinging both ways, as we don't have a negative supply rail.
The speaker is also connected to a pad, formed by R4 and R9, which lowers the audio voltage to something more suitable to drive our amplifier. IC1B is simply a unity gain non-inverting buffer amplifier, and this feeds an adjustable low-pass filter, formed by the dual-gang pot R14, R15, C7, C8 and C9 around IC1C. R15 is an output level adjust, and finally IC1D is the output amplifier, with a gain of 2 to make up for the loss in the filter. R19 is there just to limit current. C10 blocks the DC content of the output (remember it's swinging around 6V, rather than chassis) and R20 references the audio back to the chassis.
S1 is used to switch the phase if the sub makes the mid-bass sound incorrect. R11 is a 10 ohm load resistor. Some head unit amplifiers may not like being run unloaded, so it's there to give some load. You might not need it, and it will certainly do no harm to the head unit if it's not there. Add it if you need to, but make sure it's well rated to dissipate the required power.