I ran across this a while back and forgot to post it.
Hard wired FM modulator

If you have a stock CD only head unit and want to run an external MP3/MD unit, a portable FM modulator was usually your only choice. I've not had much luck with these units and they tend to run out of battery power quickly. I thought this might be a great alternative since it has RCA in's and it gets its power from the X.

I've not used this, so I can't comment on the quality, but I would hope a modulator bridged into the antenna input on the head unit would give it a strong enough signal.

If anyone has used this or similar product, I'd love to hear your thoughts.

Andy