1999 Toyota Tacoma Toyota Tacoma 2.7 L 2WD prerunner

A lot of problems commonly are caused or aggravated by forcing water into electrical connectors, but that doesn't cause failures of transmissions and things like that. Mechanics often have to do things owners don't understand, but in this case I have to seriously question your mechanic's competence.

First of all, a shifting problem right after pressure-washing the engine is not caused by an internal mechanical failure. It is electrical in nature. The most likely suspect would be water shorting out a spark plug wire leading to a misfire. You would have noticed an engine vibration, loss of power, and often badly-delayed transmission up-shifts due to having to push the accelerator pedal further than normal. Even if YOU didn't notice that shifting symptom, your mechanic should have on the initial test-drive.

Even if the engine was running properly, a shifting problem almost has to be caused by an electrical problem based on the history of the pressure-washing. To rebuild the transmission would be the very last thing to consider long after all other potential causes are ruled out.

You need to insist on some accountability from your mechanic. There are too many times when we have no alternative but to "try" things based on our experience and the car's symptoms, but in this case you need to get a second opinion from a different shop. I'm pretty sure the repairs you were asked to pay for would not have been done if this was the mechanic's own car.

Something was lost here in translation:

"he added that water was getting in thru radiator so had a transmission coolant put on separate".

This doesn't make sense. There may be a transmission cooler built into the radiator, and if that corrodes through, coolant can be forced into the transmission, but it's more likely transmission fluid will get pushed into the coolant. You'd see an oily mud floating on top of the coolant in the reservoir. The repair would be to replace the radiator, but that wasn't caused by pressure-washing the engine.

Often you just need to wait for the spark plug wires to dry off, just as would happen after driving in rain. However, if arcing took place, carbon-tracking will be left behind, and that carbon conducts electricity which leads to even more arcing. Usually you just need to replace the wires and / or ignition coils, but sometimes that carbon gets embedded in the spark plug insulators, so the spark plugs have to be replaced too.

Also look at any electrical connectors that could have had water forced through their rubber seals. That water won't dry out unless the connectors are unplugged for a few days.