First, I guess I am surprised that you don't already have a reducer. Is your cart an IQ? If not and you don't already have one, is the one you are installing a series reducer (2 or 3 wire) or a parallel "converter" (aka "reducer") with 4 wire?
If it is a 2 or 3 wire, it is connected in series and just adds a resistor that will drop the voltage to approx 12Vdc for the lights. I have not seen a series "reducer" that will work over such an input range. A series reducer would normally work be either
16Vdc to 12Vdc or from 36Vdc to 12 Vdc. The size ON/OFF switch anywhere in the series circuit would need to be 15Amp (150% of expected current). I would use (for the 36Vdc to 12Vdc or the 16Vdc to 12Vdc reducer) a standard single pole, single throw 110Vac - 15Amp toggle switch.
HOWEVER, if you are installing 48Vdc to 12Vdc "converter", it is a 4 wire parallel device. The input leads will connect to 48Vdc. Red to the positive terminal of battery #1 and the black to the negative terminal of battery # 6. The output leads will parallel connect to your light circuit. Positive to one "side" of the lights and negative to the other. (NOTE: A reminder,,,, NEVER connect any battery leads to the frame of the golf cart! NO, "common frame" ground as used in automobiles! Always run a positive and a negative wire to each accessory!) For a 48Vdc to 12Vdc-10Amp "converter" the input current at 48Vdc is a little less than 3Amps. I would probably use 14g wire on all, probably get away with no trouble using 16. You would want a SPST toggle switch rated for at least 50Vdc at 5Adc. You may need to go to a Graybar type store or search internet for Military surplus. For my 48Vdc to 12Vdc - 25A
"converter", I found a 50Vdc-10Adc combination breaker and SPST toggle switch. "POTTER&BRUMF W31X2M1G10, Circuit Breaker - Toggle Actuated - 10 Amp" on Downwind Marine site www.downwindmarine.com/Breakers-Panels-p-2-c-176.html
. I am sure if I had not been in such a hurry to get it I could have saved $ surplus or some electrical supply house. I placed the "breaker/toggle switch" between the positive terminal of battery # 1 and the positive input of the "converter". I leave it in the "ON" position using it as a breaker except if I am moving battery, doing maintenance or if being stored for a week or more (because my radio will lose memory if I don't).
Hope this helped. Good luck and please let us all know "what, which, how, where". (we like photos as well)
Thanks for serving!