That old chestnut again. I can assure you that there are components required to drive the monitor that are physically not present in radio nav versions of the nav computer. There is no way (except by re-engineering it) of converting a radio nav unit into a colour screen unit.

Sell it on e-bay again, or, if it was sold as being a colour screen unit, return it to the seller 'not as described' and get a full refund.