Passenger lights out.

I need help with my side lights and bumper DRLS.
I fitted stainless bumper with twin DRL's about a year ago, then about a month ago I noticed in a shop window that the passenger side DRL was not lit. I got out and checked when I reached work and found that the front passenger sidelight was out too.
I checked the connections and all seemed OK.
A few days later and the rear passenger light was out, then a few days later the driverside DRL was out. So I thought it was DRL related and disabled them and checked the voltage to the side lights. They were reading 5V with the multimeter so checked the wiring diagram for a 300 TDi. I then noted that the wiring diagram shows the front and rear sidelights get their power from different places, so need the experts help!
The fuses seem OK.
The light switch is a bit stiff, but would that affect both front and rear together?

Yes it can. These switches essentailly carry all the power for the lights through them. They can easily be overloaded and start to fail especially if additional lights are fitted that create an extra current draw.
Fit some relays so that the light switch only switches milliamps ....

Thanks Milktdish, but could it just affect the passenger side???

Well I purchased a new switch but without success.
I'm now going to totally disconnect the drl's as to lower the voltage to 5 Volts seems odd and may be a faulty wire creating higher resistance, through the light cable used for lights.
Is this a good idea, or am I plucking at straws?

remove the drl cable & check voltage to side lights

drl should come on when ignition is on.
Rewire with a relay using an ignition live to power the switch side.

I'll do that Hicap, thanks.

I purchased the drl's pre-assembled to the stainless bumper, and they worked beautifully for a year or so. When I noticed the problem it was just the passenger front and passenger drl affected. It then spread to the rear passenger side light, and then to the driver-side drl. The headlights are unaffected. I can understand that due to the reduced voltage, but I can understand the front being affected, but not the front and rear passenger side lights, as the wiring diagram for the Defender identifies different sources of power. Any ideas, or could the Land Rover 300 Tdi diagram be incorrect?

Look behind the speedo… do you have a pink relay?
Or look on the inner wheel arches for a large resistor looking thing.