I have a 2000 v6 blazer,It won't start .the gas gauge would go from full to empty randomly,I didn't hear the fuel pump turn on so I replaced it thinking that was the problem but the gas gauge is still not working and the fuel pump still doesn't turn on when I turn the car on,but when I turn the car off I can hear it...I replaced the fuse and relay but still won't start...any thoughts on what I should try next?

 

What brand of fuel pump did you install? It should have come with a new connector for the pump module. It needs to be soldered to the vehicle harness, crimp connectors are unacceptable. There are two black wires in the connector: both must show less than 5 ohms resistance to ground. This link should help:
https://blazerforum.com/forum/2nd-ge...eakdown-88305/