He wasn't. Dick did, however, use copious amounts of drugs.

Indeed. Meth will do that.

PKD had amphetamine problems, not methamphetamine. Though of course they are related, meth is in a different class when it comes to potency. His hallucinations and mental issues started well after he finished a drug rehab program in Vancouver so it's not certain they are related.

As an aside, the dreary, rainy atmosphere of Blade Runner was inspired by Dick's residency in Vancouver.

Was he involved in the making of the movie? Or were the dreary and rainy scenes described in detail in the book?

He was not really involved. He was consulted a bit, and while he disapproved of the original script, it seems he liked the rewrite. He screened a preview and reacted positively to the general feel, however he died before it was completed.

I doubt he was involved. I believe he died only months before the movie’s release. The story/novella describes plenty of grim weather because Earth’s atmosphere has been nearly destroyed.