When mute is de-asserted, then the ground will be cut. Well, that is, I assume the speakerphone just N/Cs the connection...would it do something else?

If it cuts it with a relay of its own - no problem at that end, but if it uses a transistor to create the connection to ground you might run into problems. Adding the diode is a low cost insurance. If they're actually Bosch brand, you should be able to pop off the top and solder on the diodes on the inside (just remember in which orientation between 85 & 86 you wired the diode [the relays actually don't care which way you wire +12 and ground across 85 & 86, so if you wire it up "the other way" with a diode inside, you'll then first pop the diode, then have a functioning relay, but w/o diode protection ])

I also assume the coil will be trying to discharge it's stored EMF back into the 12V system (ignition wire). Not sure if that's bad or not. Perhaps it'll take slightly longer to turn the amp off than expected after the mute is no longer asserted and the stereo is off?


Nah, it produces very little current, so it shouldn't affect times anything, but if you take the +12V feed from the amps feed, you might get a pop...

both the battery and ignition lines are fused, just in case you were worried.

Me worry? Nah, after all, it's not my car...

/Michael