l have just bought a 68 VW can not locate the Dimmer switch

Most vehicles made then had foot operated switches. Look on the driver side floorboard outer area. If it is not there it may have a lever operated switch on the turn signal arm. l just did some quick research. Up to 1966 they had a foot switch. After then they were located on the turn signal arm. It should move forward to change the light from low to high beam.

Pull the turn signal arm towards you.

EDIT: l have a '68 Karmann Ghia Coupe, & all the mechanicals & elecrticals r nearly the same, ''year-for-year''. Put simply, trigger the high/low by pulling the turn signal arm towards you.

My first car was a '66 bug, & the high beam was actually triggered by a separate switch WITHIN the turn signal arm. It had a choke knob & a remote throttle knob above the radio.

IIRC, those features both disappeared with the advent of the 32PICT, in '67, which featured the auto choke.

Pull the turn signal towards you and it will click. That should do it, if not there might be a button on the floorboard that you tap with your foot.

If it is a pull knob to turn on the headlights, then the dimmer is when u twist the same knob. If it is a toggle, the dimmer is the little wheel if it is a legit headlight switch. Otherwise they just used a regular toggle switch instead of the specific one for the headlights.(Same applies to the pull knob) That is for dash board dimming.
For headlight dimming, pull the signal light lever towards u about a half inch. If the headlights r on, u will hear a ''click'' . Much more convenient than looking for the dimmer button on the floor.

If u r talking about is what l know as the 'Dip' switch - i.e to dip lights for oncoming traffic then it could be on the floor (round button about 3/4'' diameter ). Alternatively, it could be on the indicator arm stalk. To dip the lights u pull it towards you, to put lights back on bright u pull it towards u again. There is no positive click that u can hear as with the later Beetles. When l was sold a switch recently, l argued with the salesman & said it was faulty as it did not move from one position to the next. But it worked.

If u r talking about a switch that dims the panel illumination - then it could be fitted to the on - off light switch & would be activated by rotating it. Alternatively it may not have this facility at all.