There are always those accessories to fit and the odd change to the electrical system to improve it beyond the basic BMW design. There are also a few ways BMW go about things which are really not very good.
These are a few of the common items people have done or are better for the purposes the 650GS is used for.
When adding accessories or modifying the electrical system always ensure the changes are documented with a diagram and put the diagram with the owners manual, too often where this is no done the details become forgotten and are difficult to follow later. I know, I have done it.
BMW wire the headlight so the main power to the headlight runs through the high/low beam switch. This will burn out especially where higher wattage globes are fitted. It is a common problem on many BMW models. It is advisable to rewire the headlight through a relay to improve voltage to the headlight and improve switch life.
Replacement of the headlight on the F and early G Series is a goal of many people as the stock headlight is not very good. Touratech have the dual head light unit but it is espensive. Others have fitted the KTM 640 Adv unit.
When setting up driving Lamps ensure the lamps are run through a relay. It is important for switch life as outlined above and also to ensure maximum voltage to the globes. Modern LED lamps improve the choices on mounting.
Wire accessories through a relay and then fuses. Modern accessories are low in power so only one relay can be used where the accessories are all on with ignition on. The best interconnection point for triggering a relay is the parking lamp wire (Grey/White) which is accessible either at the headlamp or tail lamp. It allows use of items in park.
Use the same type of relay as BMW use in the electrical box. It ensures only one spare is needed and provides a socket and location to test the BMW relays. BMW relays are simply standard micro relays but are rated to 30 amps rather than 15 or 20 amps and have inbuilt current limiting.
There are sophisticated accessory fuse boxes available at considerable cost. On the 650GS these can be overkill. The BMW 4 and 8 way fuse boxes are available through the car spares system and are relatively low cost. Part numbers are in the about section. A basic accessory wiring diagram to add your wiring to Accessory Wiring PDF
The BMW accessory socket is called an ISO/Merit Socket, it is common on vehicles in Europe. It is a more
stable and higher current socket than US cigarette sockets. It can be sourced from after market auto stores and is
ideal for battery charging but is is impractical for multiple items such
as GPS and mobile phone charging.
The 2.5mm power plugs used on plug packs work well and have been proven reliable. Powerlet are selling them as "coaxial connectors" and heated gear Mfr's use them.
An oil pressure or oil temperature guage is also possible using an adaptor on the pressure switch.
A draft pdf with info on the ABS lamp handling when replacing the dash. ABS PDF
A draft pdf with info on fitting alternative dash units. Dash PDF