Modify Voltage on Car Plug

Have you modified or can you answer me this:

I want to modify the voltage output of the LCH-6 Car plug to 3v.

The trim pot on the inside does nothing I can see on the meter, so
I'm guessing it maybe a resistor control. Without the schem its hard
to know for sure.

I have this unit in my surplus but I no longer have the phone, so
I thought I would put it to better use on my GPS.


Thanks for any assistance.