I'm new to this so let me know if I've done wrong. Where is the fuel pump relay on a 1999 mustang 6cyl?

 

under the hood in the fues relay box check your owners manual an dit will tell you which relay is which

 

I was told by a mechanic that it was behind the front bumper under the fender

 

I had to pull my fuel pump relay to replace the fuel filter on my girls '98 mustang (6-cyl). Its in the fuse box under the hood on the drivers side. Your Owners Manual has a nice diagram showing which fuse is which. Don't know where your mechanic got his information, but he's incorrect.