I'm planning to relocate to Mexico in Oct and wondering what I have to do to keep my VA card active?

Go online at VA.gov you can find more answers there.

You would have to visit the States at least once a year and go for a checkup at a VA clinic.

Your card will stay active, but you have to request a new primary care physician in the state your moving to. I've done it twice. VA will set you up an appointment with your new PCP. This can be done by visiting the eligibility office or with a phone call to VA, it's not difficult.

Your patient Veterans card? I would contact Eligibility office at your location to get that answer. Once you move to this area stop by the eligibility office here, Bldg 1, near the main entrance lobby area to update your address. Your card should remain active though.