Raspberry Pi - Transmit FM Radio
Did you ever want to play your own music on that unintelligent stereo? Well, apparently, now you can!
I stumbled across a nice guide on how to transmit FM radio with a Raspberry Pi, containing everything you need to set up your own radio station. Speaking of radio stations, do note that you need a proper license to transmit on a lot of frequencies. I seriously doubt you'll be allowed to transmit on anything remotely close to what your old radio can tune in to (88-108 Mhz).
They claim you're able to transmit on 1Mhz to 250Mhz though, so you might find somewhere to send, depending on local and national laws.
In some regions, you are allowed to transmit as long as the transmitter is weak enough not to interfere with other stations. If you live in Sweden, go ahead and read this.