I have just bought a new wiring kit for my Motorguide trolling motor (24 Volt),  the outlet has Black(-), Red(+) but there is a third wire coming from the outlet that is red and black. This set up has a jumper cable for  + to - from battery a to batt. b, black wire goes to negative to battery a red to positive battery b.  WHERE does this red and black wire go?

Ive never seen this third wire before

Here's my guess. It's the common ground. Black wire is positive 12volt and red wire is positive 12volt=24 volts. They share a common ground wire, the red/black wire and it should be considered the netural wire and goes to the ground side of your fuse block/box.

Or you could always call the 1800 number on the box...........break a leg.

I think what you are describing is your receptical is made for a 12/24 motor connection.  A 12/24 motor uses three wires and the red/black is the third connection in the receptical. Normally they use red, black and orange but your's could be using a red/black in the place of orange, color doesn't matter to electrons.   For your use on a straight 24V motor you only need to hook up the two that match the plug's connections.   On the battery, your red/positive lead will go on the positive of one battery, that jumper will go on the negative of that same battery and to the positive of the second battery.  Your TM black/negative will go on the negative of the second battery.  This will provide a straight 24 volts to the TM.  You will still hook your onboard charger (if you have one) to each of the batteries the same as if they were two seperate batteries.

I would recommend using crimp on connectors large enough to fit the wire you're going to use and then use a benzomatic tourch to solder the connections with resin core after crimping, then tape them well or use two layers of heatshrink on them.

Ben

i upgraded from a 12 volt, thats why i was confused with that third wire,  i do want straight 24.  So im guessing that i can take off that third wire(red/Black).  

Thank you for the help