Thanks in advance for taking your time to read this, I feel as if it's going to be a long question.
To start my kid has a ride on battery powered tractor and it has very low power. The battery and motor are in same under seat compartment. The tractor only works if on smooth flat surface(kid is 5) if I bypass the wire hardness it has super power, so the Chinese build I would guess used too little of a gauge wire. So my 12v goes out in to the wiring hardness powers all the bells and whistle, and then comes back in the motor on two wire which change polarity depending on what position the shifter is in. I have a bunch of 30a spdt 12v car relays I can use two where the common from each goes to each pole of motor, and make both N.C. one battery pole, and both N.O. the other pole. My problem is the coils are not polarity sensitive!! So how can I take the 2 wires coming into the compartment to switch only one relay when it's one polarity or the other if it's another polarity? I think it could be done with diodes? Or diodes and a 3rd relay? I know it's a simple circuit to some of the electronic experts here. Thanks and sorry for the long post though it would be better to explain what trying to do. I want to keep the original hardness wired in so all the sounds and forward, reverse, stop go, sounds and lights still work. It's really built in to the toy well and it would be a ton of work for me to get new wires to the forward reverse switch which would simplify which relay gets powered, but none of the built in hardness is exposed and a ton of crap would have to be taken apart to take my own wires from forward reverse switch..lol

 

Use a 3-way center-off switch from the battery to the motor.

 

I see a lack of understanding here. If the plan is to use the connections that presently power the motor to instead operate one or the other motors, then the simple solution is a diode in series with the coil for each relay, opposite polarities. Then with the shift lever in the forward position, only the forward relay will operate, while in the revers position, only the revers relay will operate. Very simple, two diodes at less than a dollar each, They can be type 1N4001, as an example.

 

So how can I take the 2 wires coming into the compartment to switch only one relay when it's one polarity or the other if it's another polarity? I think it could be done with diodes?

Any diode in series with the coil, makes it polarity conscious.

 

When A is positive 12 volts RL1 is activated.
When B is positive 12 volts RL2 is activated.

 

Thanks to all that answered. I figured I could do that, but wanted to make certain. I have to now look thru the diodes I have feom old r.s. buy when they went out, or order a package.

 

Use a 3-way center-off switch from the battery to the motor.

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Too simple, but would render the built in switches useless and make it harder for kid to stop and go, but I was thinking of doing similar with spdt toggle for relay selection when needed reverse...