Step 1: Remove 2 bolts that fasten the tail light assembly to the body
Step 2: Pull the tail light out sideways from the vehicle to remove.
See the 2 prongs that stick out from the tail light and push into the body of the van in the picture below.
Step 3: Remove the light housing from the tail light assembly by pinching the clip and pulling.
Step 4: Remove the bulb
Push in and twist.
Step 5: Determine bulb type
Printed on the base of the bulb is the type.
In my case I needed a P21/5W.
Alternatively you can look it up at the parts store. I went to Canadian Tire with the bulb in hand and I am glad I did because the parts book listed the wrong type of bulb.
Here it is in its package
Installation is the reverse of removal.
I hope this helps.