This '99 VDP is giving me one headache! I had the radiator go bad on me earlier this summer, needed the ABS pump to be fixed, and now my engine coolant is leaking. For the past year or so Ive always gotten the engine coolant low light on and topped it off, and didn't think much about it. Earlier this summer a hose that went to my coolant reservoir detached from its nipple, and that is what lead to me discovering that my radiator was bad. Last week I topped off the coolant again and didn't think much about it. The light came back on two days ago, but when i pulled into my driveway, I could smell the smell of coolant burning, so I opened up the hood and saw that coolant was spattered all over rear of the engine bay. The reservoir itself was empty, which it never has done. Attached is a picture of what is going on in there. I know its hard to see, but the coolant is green. After some searching, I think it could be an issue with the Reservoir?

 

Check your v-area for water... I had similar issues with some hoses under the intake in the v area... The thermostathousing was cracked too... Check those areas and have a look

 

From your pics of the coolant spatter beneath the expansion tank, I would first check if the lines underneath were not properly attached (pushed and pinched) as there is one hose coming from the bottom.

Best to do a simple coolant system pressure test with a 10 Euro tester you can get from a local auto parts store. You'll do this when the engine is cool/cold and will show you where the leak is located. You simply unscrew the expansion tank cap, screw on the pressure tester, pump the tester to build pressure in the system and look for leaks.

I'm thinking it is more likely a cracked expansion tank. It is just plastic and can have hairline leaks. Used replacements are very inexpensive but be careful when removing the plastic coolant lines (3 on the top) as they may be brittle due to the heat.

I had a leaking expansion tank on an old SAAB 900 and attempted patching/melting the crack back together but to no avail. Replacement for that was rather inexpensive as well.

*Edit: Daim makes a good point to replace the thermostat housing if you've not done already. It is located at the very front of your engine, is inexpensive, and takes little time.*

Hope this helps.