I think those Sony source switchers may still be hard to get. There's always the trusty Precision Instrument Electronics PIE MPSS-4 source switcher. That'll only set you back $20's of dollars, so you could skip the FM mod all together.

I think I still have the info sheet on my webspace.... Yeah, here's the specs, if that's even still available. Been working for me for 2.5 years with no problems. Good luck.