So the capacitor in my diagram is wrong - it needs to be an inductor instead. It's hard to find inductors with the sort of value you will need (and they will typically have metal cores which will mess with sound quality).

Other options include an active circuit (which uses an op-amp and needs a power supply) or some aoftware running on the car player courtesy of Hijack. I can write the algorithm for you but I wouldn't know how to integrate it into Hijack - let me know if you're interested.

In the meantime you are better off leaving out the mono-ise circuit and connecting directly. things will only go wierd in tracks whoose bass is panned signficantly, which is a minority of cases.

Cheers, John