My van will start but won't go into gear. i had the battery hooked up backwards while trying to use a different battery and i think it messed up the electronics

Kia Sedona known for sudden mechanical freeze up of transmission gear selector cable. With engine off, remove engine air intake and air filter box, look for lever on top of transmission, remove nut to disconnect cable. Install wheel chocks, set emergency brake, start engine, reduce to idle with A/C off. Manually move gear selector lever on top of transmission. If transmission functioning properly vehicle will creep forward or backward against wheel chocks. Move lever to original position. With selector cable disconnected and foot on brake, check to see if you can move gear selector from Park to any other position. If no or not easily, then either safety switch or selector cable faulty, only solution is to replace faulty part. Turn engine off, reconnect gear selector cable and tighten nut, restart and try again to use gear selector. If cannot move selector, then remove bezel from dash by hand, remove screw in ash tray, remove whole dash panel and set aside. Then with engine running watch to see if safety switch releases gear selector pin when foot on brake. If not, brake safety switch faulty. If yes brake safety pin releases, but gear selector still will not move, then gear selector cable faulty.

There is a fuse that protects the automotive systems from cases like that, I assume that you do not have a meter or a circuit testers so you going to have to remove the fuses one by one, specially the big ones, please let me know the outcome.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/flavio_517b938f50183b92

I think so as well ,think you may have cooked the gearbox computer