1999 Econoline E150 Van mirror replacement

https://www.modifiedlife.com/1999-ford-econoline-stereo-wiring-guide/

Fix your car right away by clicking the link I attached
https://www.reliable-store.com/products/1996-ford-econoline-workshop-repair-service-manual-pdf-download

Could be a lose connection in the dash , more then likely . Who replaced the gauge an drive indicator ? The horn / cruise control /turn signals / radio all share a common ground point it looks like ,G203
G203 Behind upper LH side of I/P, near cowl
Maybe after gauge & drive indicator replacement connection became lose . None of these componets share a power supply (fuse) I can't send picture of the location of G- 203 sorry ! Instrument cluster would need to come out to see the ground .

Replace bulb that is not blinking.

this is the integral small bulb that requires removal of the battery on one side and some plastic stuff on the other.. it twistlocks into position..then disconnects from the bulbs base,,

blown head gasket

In the battery fuse panel.. under the hood on the the drivers side.

Make sure there is a ground strap from the engine or transmission to the frame and/or body. I am guessing there was a cable removed when removing the trans, and that cable was not hooked back up. Alot of people forget the negative side of a circuit.

temp control actuator i heard was under the dash and you have to pull the dashboard out to replace it and takes about 5-6 hours to do it !

Under the engine, between the rear of the engine and the front of the transmission. It will be on a mount with an oil cooler, (it has metal fins) on the driver's side.