Depending on which Garmin, you may have up to 3 output connections. I am installing a Livorsi unit on mine and connecting to the Garmin 182 C.
There are 2?(I think) wires out of the back of the analog GPS which will get the power and signal from the Garmin. You should only need one antenna.
I am assuming that the Autometer is compatible with the Garmin. Autometer should be able to tell you.