where is the location of the Transmission computer

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or this is the part I think I need Toyota T100 1996-1998 Relay Integration Box Module Computer ECU 82641-34010 I installed a DVMotorsport gas saver and after one tank of gas My truck stays in low gear , and the low gear is light up on the dash , and the seat belt light stays on unless I shift into low,. And the shifter still moves through all the gears, is it this part and is there a way to test this electronic box ? ? ? ? and I am thinking the gas save module I put on cause all this ,. I feel i should get my money back from them plus the cost of what ever replacement I find, What do you think Thank Eric