I just converted my taillights to LED and I was able to get 24 volts. I replaced all the lights, including the 7 inchers for less than 200.00. If possible, can you return your 12 volt units. I was able to get amber and red 4 inch lights with water tight pigtails and grommets for 24.00 a piece. The 7 inch brake light (56 led) was only 44.00. I was able to hook directly to original wires without issue. IU got my lights from:http://www.etruckpart.com/index.htm
Good luck! I was also concerned about warranty trying to hook 12 volts to a 24 volt system? I have a lifetime warranty on these units.