Britdog15

Okay when i bought my 98 gt it had a tach and triple a pillar gauge pod. The tach and shift light work all the gauge have lights with a switch to turn it on and off. it has a oil psi installed and working the volt gauge i hooked up but the water temp has no sending unit. it has wires that are spiced into the others for the ground and 12v power. there is two red wires hooked to the water temp and one black wire. if i switch the red wires around they make the needle go under 100 or move them back and it goes over 250 with the car completly cold. Now it there a sending unit i need to buy and run it to one of the wires? or something different thanks