I don't think we need to discuss who the ultimate 'supplier' of the part is or who is liable for it's quality.

What I personally would like is for you to find out what percentage of all complaints are being caused when these crap parts are supplied and fitted. It would also be interesting to know actual figures for all the different categories of complaint.
Do you already know these?
If not - why not?
How can you defend the trade against a super complaint if you do not know facts like these.
If you have these facts why aren't they published? (I haven't found them)

As to what could be proposed, I would say this is very simple and just as easy to apply - Ask yourself what warranty comes with new vehicles - some are 3yrs/60,000 miles, some 5yrs/100,000 miles, some I hear are now 7 years. If OE parts for a particular new vehicle can last 5 years and have a 5 year warranty then "OE Quality parts" for that same vehicle should have the same warranty, otherwise how can they be of the same quality?
I appreciate that some "genuine" parts don't have warranty for as long as that, why not? Are they OE?

If this was applied I would think most replacement or aftermarket parts would have to be clearly shown to be a "cheaper alternative" with a limited warranty, minimum of 12,000 miles/12 months and this should cover all including labour without question. Otherwise label it as it is - crap!

As Neil said, All we need in this trade is someone with teeth. Who is it going to be?

Graham