Cab windows (front and side) are now modularized components to simplify changeout and manufacture. Basically it is becoming plug and play like computer components. Most EMD cabs come from Super Steel, Wabtec, or other third party vendor. In short, glass is now a largely bolt in instead of the traditional rubber seal so when a piece of glass is broken, the entire frame comes out. Also, there are economies of scale when side glass and/or both front winshields are interchangable. While I do not know this for a fact, I would expect that railroads are pushing GE and EMD to standardize on glass for interchangability and again, without measurring I cannot say for sure but the GE and latest EMD wide cabs have front and side glass that may be identical. Remeber, GE and EMD do not manufacture their own glass but buy it from a vendor. If there are no "proprietary" issues then there is no reason that EMD and GE cannot source these items from the same vendor.
With regards to light and number board placement, as stated by others earlier, it is a customer choice. The M-2 and ACE have all the wiring and resistors for the lights located in the electrical locker now instead of by the corresponsing lights and just run wire and conduit to wherever needed. Number board boxes, headlight assemblies, mirrors, cab seats, etc, etc, etc, are all made by component suppliers like PRIME, USSC, EMC, and scores of other companies. Often, class 1 railroads enter into long term contracts with these vendors for running repair parts which transfer over to new locomotive orders as well as that railroad is getting a "most favored nation" status from that vendor with regard to pricing. EMD has to offer something as "standard" but changing over these accessories to another design or vendor is pretty much the norm. Changing over these "bolt on" items is relatively easy but the days of customer specific items like high short hoods and the like are pretty much over atleast in North America.