Re: fans turn on stereo turns off....
So, my guess is that the fans are either wired incorrectly though the fuse box (or something else like the horn relay) when you want them with relays as everyone already said. Or the are wired correctly and the huge instant power draw is too much for the system, even wired correctly.
Without knowing much about those fans, my single fan can draw 50 amps at start up. Thats huge.
I think either way you should get the adjustable fan controller, that way you could run just one, and kick on the second at a slightly higher temp. And they ramp up the voltage and avoid the huge surge.
I would also upgrade the alternator to an internally regulated one, about 100 amps.
You can't go too big, it will only put out the amperage needed up to the maximum.
A one wire is easier to install, but not as accurate with the charging voltage and your 'gen' light will be on. A three wire internally regulated alternator can work the gen light and will more accurately maintain a constant voltage.
There are a couple of write up on this site on installing newer internally regulated 10si or 12ci (or something like that) alternators.