Roadster: If it helps, here's a photo of the 12V solenoid in my 40.
My old one was just clicking with a new battery so I took a chance and replaced it. If you have the equipment do as Koates suggested first. I bought this solenoid on Ebay on 5/23/2013 for a total of $11.71 and it's still working fine. Direct screw in.
https://www.ebay.com/itm/Standard-T-...Gz4vJmfjq_o8jw
By the way, take your bad solenoid apart. It's neat to see how they work and can deteriorate
from electrical arcing of the contactor.
The brown wire on the right on my solenoid goes to the alternator.