[papajam]

The lowbeam headlight relay circuit is attached. But first, a little electric 101 for those who may not know. Volts, amps and watts.
Think of these as water in a pipe where volts is the pressure, amps (current) is the flow and watts are the amount consumed. Since 1 watt equals 1 volt times 1 amp, a very useful formula for automotive stuff is amps equals watts divided by volts. For example, the Bilux headlight bulbs as fitted to series 1 1750 Euro GTVs have a watt rating of 40/45; 40 watts lobeam and 45 watts hibeam. So the stock hibeams pull 7.5 amps thru the headlight switch (45 watts each X 2 divided by 12[battery voltage]). Pretty close to the design limit of the switch. But putting in 'piss the neighbors off' 80/100 H4 paint burners will pull 16.67 amps thru the switch on hibeam and will toast the switch in pretty short order. With a relay, the switch will see milliamps instead of mega-amps since the switch turns on and off only the relay while the power to the lights goes thru the relay contacts.
Wire gauge is another thing to consider. The existing wiring may not be able to handle the extra current. The following is from the NEC.

16 gauge - 12 amps maximun
14 gauge - 15 amps
12 gauge - 20 amps
10 gauge - 30 amps

Size and fuse the wire accordingly to avoid a bill from the fire department.

Now to the diagram. The fuse box is from a US version 105/115.
Wire #1. Remove from the top of fuse #10 (lobeam from headlight switch) and connect to relay terminal #86.
Wire #2. jump from relay term #87 to where wire #1 used to be on the fusebox - use 12 ga minimum.
Wire #3. unswitched power to relay term #30 - 12 ga - inline fuse recommended
Wire #4. ground.
Your headlight switch will thank you!
High beam circuit to follow.