or just disconnect the antenna FM converters for various products have been around for decades, wired works OK....the wireless ones are junk in my experience
FM transmitters are generally crappy depending on where you are. Lots of traffic, lots of stations ... you're going to have to change your broadcast band pretty frequently.
Bluetooth is pretty flawless though.
If you want to keep your factory system, by all means do so I personally would go with a modern head unit that has bluetooth and multimedia interfaces. If you want that "original" look ... retrosound has some modern products that fit while retaining the retro look. There are also services out there that will update your OEM unit.
What they're offering in a package can be purchased piecemeal at radioshack for 30 bucks.
In all honesty, if you're willing to shell out a hundred bills for a "kit" ... shell out 50-150 more for a deck with bluetooth.