Here is a age old problem area for the following reasons; Parts mgr is most likely on a program and needs margin on the sale, The service tech wants the job done ASAP and right the 1st time, The customer wants the job done right and cheep as possible in most cases.
I've been in this industry for over 20 yrs. I started as a tech and now management.
Historically I've ran into problems using some aftermarket parts - say back when LPM was around I knew if I got a wheel cylinder or a packing kit in a LPM box, I was in for trouble. (rework was almost for sure) As a road tech I learned what worked and what didn't - The parts guy (I call librarian) only knows the part # and the margin info in most cases.

Ideally you'd select primary sources and have the part history dictate the source. In the case you let the customer decide - I'm not saying that it is wrong or out of the question.
But if you buy a poor quality aftermarket part and have to prematurely replace it. That extra 10% margin just got canceled out and you went backwards!

In most areas when options are available, I want to let our customer know his choices, I make my recommendations based on experience. If there is a gamble you try to make the customer a partner in that portion also.

I hope that helps you understand, depending on what your responsibility is. (Making bigger profits - or- customer satisfaction) that will always drive the decision unless there are standard procedures in place. Even then the rules can be bent depending on the employees.

Being that I work for a dealer, I get o.e.m. parts from my own company. However when I need parts for another type of truck, our management prefers that we go though our own aftermarket parts vendor we're associated with. I have encountered times when OEM parts were required for various makes of trucks, and all I have to do is request it at the parts Dept. and they have no objection, as long as I always check with my aftermarket vendor first.

if Im working on a hyster, lets say I need an accelerator pot. Hyster does not stock it in town. My parts dept will check and see if our aftermarket source has it. I tell the customer that I can get the part from an aftermarket source for 50.00 or I can get it from hyster for 110.00. It wii take 3 days to get here if ordered ground from aftermarket, or 5 days if ordered from hyster. The customer makes the call, and usualy decides to order aftermarket.

So then, how does the customer decided or know where parts are coming from?
I have always given the customer the choice as far as freight charges go, but I don't remember discussing vendor selections very often.

The customer always makes the call here unless we stock the part.

In my case Edward we only have two aftermarket parts suppliers in our area, so you soon work out who has what parts and what quality you get with certain parts. Im a roadie ( not the band type!!!) , and i'll generally just let the parts dept know where im going out of courtesy really. The company i work for have just acquired a major francise here so im just getting used to heading back for genuine parts for these trucks.