Thanks for the informative post crudzinskas.You are not wrong because it is certainly based on both, as time and temperature are equivalent for polymers. The reason for mileage is that miles correspond to an "at use" temperature. Depending on the location of the connection system, there is a certain temperature requirement that the part will need to meet. For example T2=105C, T3=125C, T4=150C continuous use for 3000hrs. Parts need to pass heat age testing at these temperatures for 3000 hrs with a certain level of property retention specified by the OEM. These parts are tested by many means (salt spray, temperature cycling, various automotive fluid resistance, dielectric properties, etc.. ), so that is just one test the part would need to meet.
As polymers are exposed to temperature and other environmental factors, they will over time degrade. This refers to a reduction in the molecular weight of the polymer and many important polymer properties are dependent on Mw. 100,000 miles is not that long of a time for a lot of people. But it is important to keep in mind that once these pieces are degraded, that doesn't necessarily mean "broken." In practice it comes down to the connector not being as flexible as it used to be, so if you try to engage and disengage a connector, depending on how it was designed, it may not have enough flexibility for the locking mechanisms to not break and retain the connections and terminals with as great of a force as it did when new.
As you would imagine, connectors in the interior of the car have much lower temperature requirements because the conditions to which they are exposed are much less severe.
My source is that I develop polymer compounds for connection systems and perform analytical work on components for a living . If anyone wants to talk more about polymers I can on a different thread lol!
The 1986 Saab 900T I inherited from my dad in 2000 had what came to be called "biodegradable wiring" in the Saab forum I was in. I gather the insulation was made from vegetable-based oils instead of more conventional petroleum-based stock or some such; at any rate, it did not age well and that particular experiment was abandoned after two years. The insulation on many wires in the engine bay (it seemed to vary by color) became brittle and crumbly, and would literally fall off the conductors if flexed at all! And, of course, the connectors were also very brittle and prone to disintegrate unless handled very gently, and often even then. Depending on the situation, I would either slip heat-shrink tubing on to insulate bare wires, or splice a new wire in to replace a section. Fortunately, I worked for a company engaged in aerospace work, and had scavenged a good amount of discarded teflon-insulated wire in various sizes; that stuff was great for the purpose!