Grom Audio USB & THF 25 compatibility

I'm trying to install a Grom Audio USB2 adapter on my 2006 Prius, but so far it doesn't work.

My car is the Comfort edition (which is the same as 'Base' in the US, I believe), so no navigation, no CD changer, no AUX and no cassette. The number on the front of the radio is 53815. The car came with the THF 25 (Toyota Hands Free 25) car kit installed (installation manual: see attachment) which means that the twelve-pin socket on the back of the radio was already used for a plug with a single green wire connected to it. According to the THF installation manual, that green wire carries the mute signal.

I'm not ging to use the THF kit ayway, so I just unplugged the green wire and connected the Grom box according to the instructions. Unfortunately, that didn't work. I tried switching to the Grom box by pressing the CD button, but the MFD just reported that there was no CD. Switching the car off and on several times didn't help.

Does anyone have an idea what's wrong? Is the Grom USB box incompatible with the THF kit? Should I completely uninstall the THF (which seems like a lot of work)? Or did I forget something else? Thanks in advance.

 

I did, yesterday, but I haven't received a reply yet. If they come up with a solution, I'll post it here.

 

Grom replied to my question, I'm pleased to see such an elaborate and detailed answer. I haven't tried it yet but I'll keep you posted on the results. Quoting from their e-mail:

 

I followed steps 1-7 from the email above and updated the device with new firmware. Now it works like a charm.

In the end I didn't have to disconnect the THF kit. I plugged the green mute wire into the third plug on the cable connecting the Grom device to the radio. However I have no idea if the handsfree kit still works as advertised, because I don't have a phone to use it with.