New member with some questions on '87 c-10

[djunkie]

Hello all. New to the site. Finally got a new email address that this place would accept so I could register. LOL!!!!
So I picked up a super clean '87 last October and I'm fixing some little things on it.

Ok first question. I got a speedo issue. Truck is powered by a 350tbi with a super trick built 700r4. When the truck is cold the speedometer works for about the first 2 or 3 miles miles then it bounces its way down to 0 where it stays till the truck cools off.

Second question. One of my clear side marker lights was cracked when I got it and a piece finally fell off. Where can I get a replacement? I'd like to find the same exact style. Thanks for your help. Heres a pic of the marker light and a couple of the truck.