I know it's not a lot to go on, but I remember my parents having 1 VHS on an American voiced cartoon that featured a giant Gundam-style robot that had a big yellow "X" on black plate either on his chest or shoulder plates (or possibly both). It wasn't a very complicated looking mech either
Due to the extremely foggy memory of a time when I was so young I can't be 100% sure of exact details but I'm almost sure their base was in a mountain or something. Also played the Axel F Beverly cops song when he jumped in and the robot was flying out if I remember correctly.
This could be a very small-time cartoon that wasn't very well known as I've been searching over the Internet for a while and never seen a description or screenshot that matches but I'm hoping an enthusiast may come across this that knows what my brain is trying to piece together!
Don't restrict suggestions too close to my description... I do remember the "X" was there, but my memory may be playing tricks with me on other details.