The wiper system would probably be the first thing they need to look at.
Add a brake light system. It's just a micro switch on the pedal, wire leading to a brake light mounted in the rear. It doesn't need to be a full light cluster, but drill 2 holes in the rear around the decal area and just stick a brake light there.
Make the rule like F1, if it rains then the rain light needs to be on. If the track is dry for Nascar, no brake lights and the racing can continue to be traditional.
But honestly, in this day & age... Nascar has no excuses for not racing in the rain at RC. They have the tires, they've got the knowledge from all the years racing up at Montreal, they just need to be smart about it.
The rest of the racing world sees F1, IndyCar, IMSA, V8SC, Touring Cars and GTs series racing in the rain.