I am sure you did, but since then laws and regulations have changed drastically in Thailand. Nowadays you need permits and heaps of paperwork done in advance and you need guides as well.
Although as I wrote above here - some border crossings seems to not enforce those new laws and regulations. But I wouldnt count on that I would find such a bordercrossing both on the way in and the way out.

Remember also that there are limitations of what cars the thai DLT (department of land transport) allow into the country. Not above 3500 kilograms, no campervans, no cars with permanent bed/sleeping arrangment etc etc.

I will add a posting from the company Travel Active that they posted recently on Overland Asias facebook site:

"We have recently had an influx of requests for Thailand permits at short notice.

Generally permits take 20 – 45days to process, and we would recommend applying 2 months in advance.

I just want to explain the time frames and process:
• We collect and review all documentation
• The application is forwarded to the DLT for initial checking
• Normally the DLT responds within a week to confirm whether the information submitted is satisfactory.
• Further to this the DLT processes the permit which can take a further 2-5 weeks.

Updates:
Currently the DLT is not providing permits for / allowing vehicles entry which are Motorhomes, Campervans, vans, have a van body shape, have permanent bed or are over 3500kg. Photos of the vehicle inside and out are required with each application.

Basis of our Fees
We are running this service on a low margin to assist our members (far below what we would normally charge if it was being run purely for commercial reasons) – We do make a small margin on each tour however its very unlikely we will recover the costs of investment into the service.

Please be careful when choosing an agent, some have proven to be un-reputable.

We are owned by the same people who run this forum, Overland Sphere and Overlanding Association, so our interest is in helping you cross Thailand or other regions in the most cost effective way possible whilst maintaining reasonable standards"

__________________
In the end everything will be fine. If its not fine its not the end....