Not wanting to spend the money for a Zumo when I already have a Nuvi 1450 I needed to figure out how to get the voice to my helmet speaker. The 1450 does not have an earphone jack.
I had previously installed an $8 Radio Shack speaker in the left side of my helmet with a panel mount minijack in a 3/16" hole I drilled in the helmet. This was for the radar detector.
Needing to be able to hear both the radar detector and the GPS I bought an AmpliRider from:http://www.electric-avenues.com/amplirider.html
This was $65 delivered. I then bought a self powered mini mic from:http://www.soundprofessionals.com/cgi-bin/gold/category/106/mics
This was part #ATR3350 for about $25 delivered.
The amp allows up to three devices to be mixed into one output but has no priority channel so each device has to have it's volume set. The amp does have a master volume control. The mic is battery powered(included) and has an on/off switch. The battery is supposed to have a 200 hour life.
I velcro attached the mic to the back of the GPS unit at the speaker and shielded it with a small piece of foam for wind noise protection.
Power for all three devices(detector,GPS and amp) is from the dash outlet.
The detector output is connected to the amp input. The mic is connected to the amp input. The amp output is connected to the helmet speaker.
I have bench tested this setup and it works very well. It should have all the volume needed. I will road test when weather permits.
This setup gives me an inexpensive means to use a lower end GPS and my radar detector at the same time. Sorry I have no means to include pics.