My car wll not start in the rain I took it t a mechanic and had a new distributor cap put in , I see no o ring around the cap. What Shall I do ? the car is a 1995 cougar

May be a cracked or poorly fitting distributor cap.

I don't know what the mechanics did or did not do, so some of this may be a repeat.
Electricity takes the shortest route to ground. So any moisture can act as a conduit.
In addition to what you listed, the ignition system also includes the pickup coil and module inside the distributor, the rotor that provides a link thru the dist cap, the wiring and connectors in the harness on the engine, various relays and other components.
I would make sure the dist rotor is new, then try to duplicate the event.
Take a spray bottle like Windex and spray the area around the distributor first with regular water, then try to start the truck. If that does not cause the effect, try a garden hose and gently spray the rest of the engine compartment, but don't get any water inside the air cleaner or the intake.
Once you get it to not start, you need to trace the ignition system to find out where the short to ground is.
It could even be in the harness for the fuel injectors.

change the distributor cap by friend did the same thing and i wouldent start untill we took the distributor cap off and we seen croshin and we replaced it and it started up like a champ

does that truck have a distributor cap? if so then the moisture of the rain is causing it not to fire most likely. replace cap or next time it rains take cap off and dry it with a blow dryer inside the cap, without removing wires and if it starts when you remount the cap then replace it and seal around the cap and distributor as well as use electrolite grease on the contact points to repel moisture

if it does not start after you check your distributor cap that means you took the car out of timing, when you put the distributor cap back on you did not put it is the correct position, now you need a mechanic to get the timing of the engine

First, I checked to make sure all spark plug wires were o.k.. There wasn't any visible cracks in the top of the distributor cap. So I took an old hair dryer and plugged it in. Set the hair dryer on the top of the distributor cap for about 10-12 minutes. Thats enough time to dry the inside of the distributor out. Engine starts fine so obviously there is a hair line crack somewhere on the distributor cap.

check the distributor, there should have a seal ring where the cap goes. the ring should be inbedded in the distributor housing.

Time for a complete tune up, this time put a distributor cap on too, plugs wires cap rotor, and use quality parts. what you got here is condensation getting under dist. cap most likely because cap has a hairline crack in it.

Probably distributor cap is worn. Moisture condensed inside. To check it heat it up with hair dryer for 10-15 mins. Try to activate the engine
If it starts look the problem where. If not check spark plugs/cables.
Ignition control module
Good Luck

Sounds like moisture is getting into the distributor cap. Try putting a thin film of petrolium jelly around the lip of the distributor cap where it seals down onto the distributor. This should stop the water from getting in.