"On the firewall just to the left of the brake booster" would be the glow plug relay. The cold start solenoid is in the Injection pump, under the pump housing cover. There is a green wire to the rear most connector, it also connects to your fast idle. The pink wire is your shut off solenoid. The sensor is on the rear passenger side head, follow the wire. Does your truck engage high idle when cold, should sound rougher as the timing is advanced 6 degrees. If it is your temp sensor, be careful not to over tighten it, or it will snap in half. Long ago I wired an illuminated rocker switch to the circuit, bypassing the sensor, so my system is completely manual.
