donatas, I'm sure you could use pin #48 going into the ECU. The Autometer is just using the voltage of the O2 sensor, which ends up going to the ECU. Check out the tech article over on the MKIV.com for the S/AFC. There's what's called the "Blue wire mod". This enables the S/AFC to monitor the voltage being transmitted from the main heated O2 sensor.
If you don't want to try that then just tap into the main one O2 sensor. I was thinking about getting an Autometer but I'm shooting for the Techtom or the other one (can't remember the name at the moment, getting old is no fun, all the parties along the way were priceless )