It is possible that radar jammers existed even before radar detectors did. In the June 8th, 1953 article of Time Magazine, there was this little blurb in the "Miscellany" section:
"ELECTRONIC COUNTERMEASURES - In Jackson, Miss., highway patrolmen were looking for the motorists suspected of building short-wave radio transmitters to jam the police radar speed detection system."
Early literature on the subject of countermeasures is sparse. One of the first major articles mentioning the possibility of radar detection was probably the May 1956 issue of Popular Electronics. The cover article was titled The Truth About RADAR Speed Traps.
The article is in a question and answer format. It goes through some of the technical details of how a "Radar Speed Meter" works, and also dispels some of the gimmicks and rumors of ways to defeat detection, such as mounting an "absorbtion shield" behind the radiator, grounding the car through the hubcaps, or scattering aluminum foil. One other interesting note, is that the article mentions that at the time of writing "The state of Ohio requires a notice be served on the public not less than 750 feet in front of an operating speed meter" Aaaaaah, how times have changed...
The article goes on to discuss the possibility of jamming or detecting the radar speed meters. It said that jamming was possible, although illegal, however licensed hams could "accidentally" jam the speed meter by operating on the same frequency. As far as detectors, it had this to say:
Question: "Couldn't a microwave detector be built to warn the motorist?"
Answer: Yes, a detector could be built, and several designs were considered during the preparation of this article. All of them are costly to build and have limited range.
The article went on to say that if there was sufficient reader interest, they would task a project enginner with designing one so that the design could be published in a future article. As for the future article: it appears that it never happened.