Doable? Sure.
A lot of that depends on your tuner I would say. With LMS, they have ALWAYS recommended the use of 100% Methanol. So from that vendor alone, you would not want to comingle the two.
I would also purport that tapping a line off of/from the OEM washer reservoir, and using that one tank, for both purposes is also less than desirable overall.
Why? Well all I can say is that once your on the meth train, you'll be surprised how much you actually go through.
You'll want to retain that entire volume for exclusive use of your meth system alone.
Having performed this install myself (with the help of a very close friend who is a master mechanic) I can speak from experience that running a 2nd smaller tank for washer fluid, and utilizing the OEM tank for the meth system is really not all that difficult of an installation.
Tapping a 2nd line and drilling holes into your OEM tank as your sole source is going to be more tedious as you will have to ensure no leaks result. Just seems like more work.
So all in all, sure anything is doable, but what's better is completely subjective. One must look at their skill level, their intended use, and parlay that with what your tuner recommends.
Make sense?
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