The 75X series is a bit like all the different 8051 variants: the same basic "core," with various different peripherals/extensions.

You said this device is in your car hifi, so it's probably mask or OTP ROM and you won't actually be able to do anything with it.

To just learn of the structure, pick a datasheet from one of the others which is on the website; eg,

uPD750064 Data Sheet:
http://www.ee.nec.de/web_cd1/p...V0DS00.PDF

uPD750064 User Manual:
http://www.ee.nec.de/web_cd1/p...V1UM00.PDF

One interesting feature that I remember was that they don't have any conditional jump instructions; only conditional skip (skips the next instruction if the condition is true).
Thus, to do a conditional branch, you had to do something like this:

skip if condition
jump             ; skipped if the condition was true
more instrucions