It is getting hard to find decent
cassette adapters now that most car stereos only have CD players and come with aux jacks. The best I ever had was from Discwasher, but I doubt it is still available (not sure the company even exists anymore). Also got a good one with a Panasonic CD player that I still use.
For a while there was a Bluetooth cassette adapter, but it just did headset profile, not Stereo Audio. There was a review and some advertisements for a stereo one, but it never made it to the US market (was available for a while in Korea).
I am currently thinking about just biting the bullet and getting a new head unit. This one looks pretty future proof:
Dual XHD7714 CD receiver at Crutchfield.com
Has CD player, HD Radio, regular AUX input, Bluetooth, and a USB port so you can just have your music on a USB stick or portable hard drive, or could plug your Droid in and mount the SD, and it also works with Ipods.
Alan