fender bass cases

Fender p-bass?
Ok so heres the deal…i have a 1985 fender p bass…i want to mod it out..i have already bought a new bridge b/c for some reason the saddles on it dont stay where they r supposed to and the screws wind down by themselves…also im not sure but is this possible…can i have route out a noe hole near that bridge and put in an active jazz pick up and then put in a new pot and 3 way toggle switch so that i can have it either play just precision, jazz or have all three pickups working at once?..or do i have to put in another tone pot along with the one ill put in for the jazz pickup, making it a total of 4 tone knobs, two pbass pickups a jazz pickup and a 3 way toggle switch?…and also am i overlooking any other mods i may have to make to it other than a battery casing?
i want a bass that i can play as a pbass and a jazz bass…the toggle switch being to switch between either p pickups, j pickup or to have them pickup at the same time.
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1961 Fender Bass VI 01056