I am installing a new TM battery. After looking at instructions I am a bit confused. My current setup is 12v, with one battery. I am questioning the way it is currently wired. There is 2 large red wires that are identical (not orange, yellow or striped) attached to the positive battery terminal and 1 large black wire attached to the negative terminal. I read it should only be 1 red and one black, but which red do I use? The TM recepticle at the bow is a 3 prong plug. I removed it to check the connections on the back side. 1 large red wire is connected to the top by itself. 1 large black and the other large red wire are connected to the bottom connections across from eachother. I assume this was messed up from the previous owner of my boat. Which red wire should I use, and which should I cap off? Does it matter? Thank you for your help.

Doesn't matter which one is capped off long as the red from the TM matches up with the 'live' connection on the plug.

 

I'd personally replace the 3-prong TM plug with a 2-prong one...but that's me.

Sounds like your boat is from back in the 80's with the old 12/24 system.  If you want to do yourself a big favor, get rid of that AC outlet and plug they used. If you look, it's probably only rated for 30 amps and they are bad about overheating and melting things with these newer TMs that draw way more than 30 amps if you run on Max very long.

Usually, they have two red cable and two black cables going from the battery to the bow.  The first thing I do when I come across one of those is buy me a pair of SB50 connectors and do away with all the crap at the bow. 

Put both of the red wires on the positive terminal of the battery, going through a 50-amp breaker to do that.  Connect those same two red cables to the positive lug on the SB50. 

That gives you the equivalence of having one size larger cable, which is better than just one.   Connect the black cable to the negative size of the SB50 and negative battery terminal. 

Put the other SB50 on the TM cable and just plug the two SB50 connectors together, doing away with all that junk the gets hot and reduces performance of the TM.