I did this the other day with my HD1BT, I assume it is the same method
Soft reset using the reset hole and stylus provided
then once booted, turn your engine off until unit is off and unpowered
then hold the reset in again
turn engine on, and when its booting up, hold down the map key
it will then ask you whether you want to load factory defaults
this will wipe all settings including the phone (Which is why i did it )