(T) Warning light (YELLOW TRIANGLE) and radio code

2009 Diesel Fiat 500, 57k miles, fully serviced etc. Does probs 40 miles/day

Drove fine on Friday - to and from work
Drove fine on Saturday (only a couple of miles)
Started up first time this morning but had the yellow triangle warning light on and the radio was asking for the code (like it had lost power over the weekend) - car drove fine and no other warning lights, all bulbs seem to work fine (usually display a message to say a bulb has gone)
the demister/aircon dials sometimes light up/sometimes don't when the headlights are on - so potential wiring issue

Any ideas/where to start looking?

needs a new battery

Is that a definite?

if its your stock battery that came with the car then yes defintiely change the battery. If you go to a nice guy to change the battery, have him plug in diagnostics after to see if the problems have gone for good.

This is what i did as i changed my battery at the Bosch Service Center and they checked it with diagnostics once they had done so which showed no problems.
When the battery was dying, several messages had come on, including Engine Check Light and Power Steering Light. These all disappeared with the new battery.

All sorted thanks - new battery and one of the rear brake lights had gone :-)

cheers fellas

Thanks for coming back and confirming that the advice given solved your problems. It's good manners (all too rare) and also builds the "knowledge base" of the forum for people who are searching for the same issue in future.