Hi, glad to hear the ones you got have worked. LEDs are polarity sensitive, so will only work one way round. This is normal - unless there are some clever electronics in the housing.
I don't have any close ups of mine as the camera cannot actually focus on them when they are on. Will take a pic in daylight, but basically they are just stuck to the lip of the crossmembers where they meet the roof. The suggestion of the original "transit" ones I bought was to stick them to the bottom of the crossmember but I thought about it and decided that put them in harms way and likely to be bashed at some point, whereas now they are protected.
Definitely worth doing longer term. The white panels do help - but even without it is still very bright. I painted the panels mainly because they were badly stained and looked a mess when I bought the van.