The sun will rot most, if not all plastics, with PLA, ABS and PETG rotting to varying degrees and at differing rates. So, if even part of your enclosure, such as the edges, are visible, they will invariably be exposed to the sun at some point (although maybe not sufficiently enough) which will make them brittle.
However, as you say, your main concern here is not direct sunlight, but heat. As this informative article, Using PLA for Long-Term Outdoor Applications, suggests:
PLA is great as the warping is less than with ABS.
Interestingly, it also notes (again related to sunlight, not a concern in your case):
As a side note, PLA is referenced as considerably UV resistant.
Whilst your question is not a duplicate, as such, there are already a number of questions (and answers) on this topic, that I remember having seen and being relevant.
Whilst I have quoted some of the relevant parts below, it might also be worth taking a look at the other posts on these links:
Keep in mind that PLA has a much lower temperature point, where is starts getting flexible. I once had PLA-printed parts in my car in the summer for 3 hours and when I came back, they where bent.
I don't know about the weather conditions in your local environment, but if you experience hot temperatures and your sign is hanging in direct sunlight, I would suggest to make sure you secure the letters against bending (e.g. cover them with a coat of epoxy or something like this).
The property you're looking for in the thermoplastic (which will determine the continuous operating temperature) is glass transition temperature. This is the point at which the plastic begins to flow, and becomes deformable as EvilTeach described. PLA reaches this state at around 60°C, whereas ABS is around 105°C, just suiting your specifications. To go a bit further, polycarbonate offers a glass transition temperature of around 150°C, and Ultem at 217°C.
50°C is hot for you. PLA's glass transition temperature is 65°C. A car in the mid-day sun can get very hot indeed.
... if the part is designed to be strong enough for its use in PLA, it will be no better in "stronger" ABS. If the PLA part will be "more precise" and "less warped" - that may well make it better for its use. Other than a widespread community dedication to self-replication, (or mostly self-replicating with some metal parts) there's plenty of arguments for making most printer parts out of machined metal, for that matter - much stronger than ABS or PLA.
PLA also will slowly melt in direct sunlight. I have seen this one firsthand, having left a print on my windowsill and watching it slowly morph with the weight of objects on top of it.
Also related, temperature wise, although not strictly in-car related (although that is where you would most likely find a travel mug), is the most informative answer by Ryan Carlyle, which makes mention of annealing as suggested by OyaMist Aeroponics's answer:
It is possible to anneal PLA to survive higher temperatures, as this increases the crystallinity of the polymer and thus makes it more heat-stable. However, that is highly experimental and results will vary considerably with filament provider, color/pigment, and annealing process used.
PLA would be a non-starter for outdoor use as it's biodegradable and can breakdown in sunlight. Albeit slowly, but won't be useful for long term project.