Switch phone mono line off for mp3 use

My Sony Ericsson phone (810i) also plays MP3s.  I have a phone kit that I can hook up to the HU in several ways:

1) Phone stereo audio to HU aux stereo input.

Advantage: Music plays in full stereo through car sound system.Disadvantage:  Phone calls can only be heard if AUX is selected as the HU input source. This includes both the phone ring and conversation. Phone calls could be missed if other sources(CD, FM, etc.) are in use when the call is received.

2) Phone mono audio to HU mono phone input and phone kit mute to HU mute sensor.

Advantage:  When listening to CD, Sat., FM, etc., and a call is received, the HU will mute the current source and the call will be heard through the HU sound system (mono).Disadvantage: MP3s played through phone will be in mono.

Note: The mute line on the phone is activated regardless of a phone call or MP3 use.

I would like to use #1 when using the phone as a MP3 player and #2 for all other times.  I know I could build a manual switch to turn off mono & mute inputs & turn on stereo input to aux input for MP3 listening.  But, what I would like to do is somehow sense the AUX input is in use (i.e. I select it as currect source from the HU), and if so, automatically switch off the line to the mono phone input & mute input. When I select any other non-AUX HU source, then the mono phone input & mute are switched back on & calls can come through.  Any ideas how this can be done?  Can this be done with relays?