Controlling a vehicle gauge cluster via CAN

I'm playing with an instrument cluster from a late 90s Ford Ranger, and using an Arduino to represent the rest of the vehicle. Triggering individual indicator lights via a transistor IC was easy enough. I'm starting to work with the speedometer and tachometer now.

The speedometer works by receiving a pulse signal from a sensor in the transmission. I've determined what frequency represents what speed using my function generator. The issue is that the gauge pulse signal seems to only affect the gauge needle when the voltage goes negative, which an Arduino only supports 0 to 5 volts.

Does anyone know how to trigger a negative voltage at a variable frequency?

I've been recording my progress here:
http://jason-joyner.com/can-to-factory-ip-cluster-converter-module/

That links gives an overview of what I'm trying to do.

"The issue is that the gauge pulse signal seems to only affect the gauge needle when the voltage goes negative, which an Arduino only supports 0 to 5 volts."

When my multimeter reads negative voltages, it's usually telling me that the leads are reversed.