When you replaced the rotted out relay in the relay box, did you inspect the wiring going to the relay sockets to ensure that wasn't rotted out as well?
Easiest thing to check (relatively speaking) is the starter...it's towards the front of the car. The starter consists of a large cylindrical body with a smaller cylindrical body on top of it, with a thick bare copper wire running between them (it's very short, like 3/4" long). Look to see if that wire is intact...they have a tendency to corrode away. If it's rotted away, replace the starter. If that wire looks okay then give the larger starter body several taps with a hammer, then try starting the car. If it starts, your starter is bad.