How can I symlink the /cache folder on boot?

After rooting and installing link2sd on my **HTC Explorer**, I still have low memory issues. On Google Play, I can't download apps larger than 10-15M. This is because the /cache folder is in the internal memory, which is desperately small. In my version, Google Play writes directly in /cache, not in subfolders. Using adb as root, I deleted /cache and created a symlink /cache -> /mnt/sdcard/cache. This is fine, but when I reboot the phone the symlink is replaced by the old folder. Is there a proper way to solve this issue once and for all? Thanks for any hint, A