Classic/Antique Car Repair: 1987 Chevrolet Custom Deluxe Blazer 4 x 4, fuel pump relay switch, oil pressure sender

Question
I have a 87 blazer 4 x 4  having problems starting and dying out. I replaced the fuel pump and at first this seemed to help. Ended up replacing fuel pump 1 more time truck still does not stay on. I was told it may be the fuel pump relay switch. Where is this located?  i was told it is by the firewall but i cant for the life of me find it!  was also told it is behind the glove box but nothing.  Does this year of truck even have a relay switch and if so where is it located at?  truck acts like it wants to stay on but dies out.  thanks for any help you can give.

Answer
The problem may and may not be in the fuel pump relay. It is located on the right (passengers side of the fire wall right over the heater blower motor of full sized truck and on the left inner fender panel on compact (S series) trucks. The relay is controlled by the engine computer when turning on the key and cranking the engine. Once the engine starts the oil pressure sender switch reads the engines oil pressure and takes over from the computer to keep the fuel pump running. This is a fail safe system that will shut off the fuel pump if the engine should stop running like in the event of an accident. On these computer controlled vehicles it is cheaper to test the system and find out which is defective that to just start changing parts because somebody said.
Brad