2000 Chevy Astro Van will not start when it Rains.

I am sorry but I am missing a lot of details. My main focus is how you guys handle this problem? Here's the story: my friend has two Chevy Astro vans and both of them have the same problem. I never caught the problem because whenever I see the van the weather is nice and it starts with no problem. I did spray water onto the engine and watched the ignition system but no change whatsoever. I did replace spark plugs, spark plug wires, cap and rotor, ignition coil on both vans. On one van I installed factory coil and spark plugs. It is ok but only for three to six months and it goes back to the same problem again. When I clean up the distributor cap it gets better but for a short time and then it comes back again. When it happens, the owner opens the hood and blows hot air from hair dryer towards distributor cap and it starts with in 5 min. with no problem for the rest of the day, even though it rains the entire day. I remember I did not have any DTCs and the engine was running good. It only happens on rainy days (when it rains for 2-3 days or more), it has no problem cold start (whole winter it starts ok). I did not have the scope at that time, and I am starting with the scope right now, but I'v seen this problem in other vans. It seems to me that this is a common problem with this engine. I'v seen one video about it advising to add extra ground cable to ignition coil bracket because the bracket is mounted to the intake manifold and there is corrosion on the bracket mounting bolts which cause weak ground to ignition coil housing (perhaps I will be able to see a weaker spark on the scope?). My question to you is:
1 How do you fix this problem?
2 Did you notice a difference in quality of distributor cap and rotor (I am thinking factory part vs. aftermarket)?
3 What could cause the distributor cap wear out faster?
4 Is there any aftermarket manufacturer who makes better ignition related parts?
5 What to focus on during scope testing with this type of the problem?
Sorry if it is too long or too many questions. I am new to the forum so I may need to be corrected.
Thank you very much and I hope that this is an interesting topic

A problem I have encountered a few times in the past on different types of vehicles, there are two ways I have dealt with it.

The first is to use an ignition sealer nb. not a water repellent but a spray on flexible laquer. I don't know the brands sold in the USA but I found this on EBay US link Make 100% sure the parts are 100% dry before applying. However it is best applied to a new cap.

The second line of attack probably more effect in the longterm is to shield the cap and coil from the source of the moisture, it dosen't have to anything elaborate you can improvise with plastic sheet or a polythene bag.
I don't know the Astro Van but from Google picture I see the distributer is mounted high at the back directly under the bulkhead so I would look for a way to stop water running down on to it. Also check a heater air intake drain isn't running on to the ignition.

Battery is 1.5 year old.
Thank you for your replay.

I have experienced similar issues.

My Dad taught me the trick of spraying WD-40 into the cap; this displaces the water and gets the car going again.

So, IMHO, it appears overnight that condensation is forming in the cap. So when the car starts up, it causes the condensation to turn into droplets big enough to cause cross fire.

So, I solved the issue by purchasing a cap with a little hat on it that allowed warm air to escape the cap and not cause condensation.

In regards to secondary graphing, when you graph it at idle, all should look ok. But, when you perform a snap throttle, you will notice that the spark lines won't be as high on the plugs where shorting is occurring. This is because the cylinder pressures aren't affecting the plug wires shorting in the cap. Sort of hard to explain, but really simple to show. ScannerDanner has some great videos on this spark line analysis.