You're in a difficult bind. As far as I know, there are no car services which would take you from San Diego to Tijuana. In addition, no major rental car company in San Diego will permit their cars to be driven into Mexico and they will be certain to remind you of this at check out. This is primarily due to insurance concerns. Taking a rental car to the border, parking it in a secure lot, and then walking into Mexico is a possibility if you don't have a lot of "stuff" to carry with you. I would call the hotel (I hope you are staying at the Marriott) and ask them for their advice. I guess another possibility would be to fly MX or AM to Mexico City, transferring to Tijuana and then taking a taxi to your hotel. I've used the Tijuana Airport many times and found it to be clean, safe, and easy to navigate. Good luck to you.