They are located at the spindles. Just remove a wheel and check out an electrical connection. ABS sensors must be replaced when defective but it seems you don't know what is really defective. It could be the citcuit, one sensr, or the ABS motor or whatever. It couls also be low fluid. On my car we used the scanner from Mitsu at the garage and it showed what sensor was gone. No problem, Mitsu fixed it for free in 10 minutes (but this is Europe !)