make neons wigwag with music?

I posted this in the relay section last year, but it was never answered...

I need to replace a SPDT relay with something that is silent.

here's the background:
I have a Streetglow Music Interface and a lot of blue and turquoise neons.  The Music Interface has an on/off switch and a music/light switch.  When the switch is on "light," all the lights are on, constantly.  When switched to "music," it gives off a constant positive(+)  signal to the neons and the negative(-) signal switches on and off...on when there's a beat, and off when it's silent. 

If you add a SPDT relay to the negative output of the MI, you can have the neons flash alternately.

The problem is that the relay clicks loudly.  Not only that, it clicks once or twice a second.

"Thinkster" from the12volt.com drew this out for another member with the same idea.

The problem is that it has 12v+ outputs.  I need 12v-.  I've been trying to email him for months but he's never gotten back to me.

I plan on using another switch to choose between having both the blue and turquoise neons on at the same time, or having them flash alternately.  I want to still be able to control the neons normally from the music interface.  

Here's what I want to do...

I changed the way the relay is set up...

When the MI is set to flash to the music, and the blue neons flash, the turquoise one should not.  And vice versa.

Can somebody help me come up with a way to replace the relay?  I need something that is solid-state (silent) that will also handle about 6-10 amps.  SPDT solid-state relays are out of the question since they're ~$300