A temp sensor
The speed sensor screws into the side of the trans/axle on earlier Intrepid models, I would assume it's still there on a 99..... Yeah, I found the speed sensor, it was located on the back of transmission. Had little difficulty accessing it, but once replaced it, it fixed the problem.
In the fuel tank.
It can, but not always.
your switch is going out....you may need to replace it
it doesn't have one
yes
There is no fuel pump inertia switch on a 1999 Dodge Intrepid.
It controls the operation of the transmission.
ECM means "Engine computer module"
Yes they will , just change around the valve covers and intake from the new to the old.
If your car is anything like my 2001 intrepid, it is because only one of your radiator fans are coming on when the temp sensor tells your fans to turn on. You hear a fan kick on, but it is only one of them. There are two.