My 6v regulator was found underneath part of the casting that supported the drive... turns out that it was a 3 terminal regulator, and you could not see it from above. a wire went from the head of the loco, through this structure and back out. what made me nuts was how it got converted to 6v... thought it was just a wire going through, but turns out the regulator was grounded to the metal, so while it looked as a single wire going through it was one in, connected to the regulator, then the one out was on the regulator output. Elsewhere the ground to the chassis was found, that supplied the 3rd connection to the regulator... very tricky!
I actually found it by feel, following the wire with my finger, and finding a lump!
Trust no one ha ha!
Seriously, do it bit by bit, all the while checking that the wires you are using are truly isolated from others, I start with the motor, then speaker, then headlight, etc. one by one.
Greg