Climate control display problem

Hi Guys

Recently I bought a Climate Control Unit (Second Hand) for my Superb as the original unit was knackered. (Fully illuminated, but readable data made no sense)

I fitted the new unit to the car and it worked perfectly, read perfectly, but did not illuminate.

I decided to purchase another second hand unit but unfortunately this did not illuminate either, although it worked perfectly.

Both sellers confirmed that they fully illuminated prior to the sale. (Both Accepted Returns and are Reputable Sellers)

Any ideas / remedies ? Some one told me to recharge the gas but I dont see how this would impact on the illumation of the unit ?

Lindsay