The marker lights are part of the Ram Promaster Van and were installed by RAM.
You get them fixed or replaced at a RAM dealer (preferably a Ram Businesslink Dealer who sells and is experienced working on them.
Note: This has been a known issue and TSB’s (Technical Service Bulletins) have been issued on it in the past.
(These are not a Winnebago installed item and Winnebago makes no changes to them.)