Try this.
First keep in mind as Hacked89 is alluding to, the solenoid on the fender well just sends power to the starter soleniod. Yes you have in essence two starter soleniods. One on the fender and one on the starter.
So to test the starter do the following.
1) Use a multimeter to test the batteries. If less than 11v then charge them.
2) Take a battery jumper cable or screw driver and connect the two large posts on the starter soleniod on the fender well together. You will get some some sparks but that is normal. If the engine now does not turn over do the following.
a) Check your battery connections and grounds.
b) Pull the starter and have it tested. If it tests good see if your engine is locked up. If not recheck your connections.
3) If the engine turns over check your wiring to and from the solenoid on the fender well and to the starter.
From there you will know if you have a wiring issue or a starter issue. Post some pics and let us know what you find.