Coope said:
......... A colleague at work who is going to give me a hand has asked why use a relay ?
I have to say with questions like that should you/he really be doing the job? :?
Please don't take offence as none is intended but not getting the electrics right can end with a melt down of the wiring or worse
However in answer to your question, if you wire the DRLs direct to an ingition switched feed whether it be from the heater motor or anywhere else you run the risk of overloading that circuit and the fuse to that circuit may keep blowing and/or the wiring start to overheat.
Using the power to energise a relay however should make no noticable difference to a circuit like the heater motor circuit as it's already fairly beefy.
If both DRLs on at the same time draw less than 0.5A you will no doubt get away with powering them directly from the heater motor but if a job's worth doing it's worth doing properly... fit a relay or don't do it at all :wink: