Context: The methyl cation (CH3+) has recently been discovered in the interstellar medium through the detection of 7 µm (1400 cm-1) features toward the d203-506 protoplanetary disk by the JWST. Line-by-line spectroscopic assignments of these features, however, were unsuccessful due to complex intramolecular perturbations preventing a determination of the excitation and abundance of the species in that source. Aims: Comprehensive rovibrational assignments guided by theoretical and experimental laboratory techniques provide insight into the excitation mechanisms and chemistry of CH3+ in d203-506. Methods: The rovibrational structure of CH3+ was studied theoretically by a combination of coupled-cluster electronic structure theory and (quasi-)variational nuclear motion calculations. Two experimental techniques were used to confirm the rovibrational structure of CH3+: 1. Infrared leak-out spectroscopy of the methyl cation; 2. Rotationally resolved photoelectron spectroscopy of the methyl radical (CH3). Results: The quantum chemical calculations performed in this study have enabled a comprehensive spectroscopic assignment of the ν+2 and ν+4 bands of CH3+ detected by the JWST. The resulting spectroscopic constants and derived Einstein A coefficients fully reproduce both the infrared and photoelectron spectra and permit the rotational temperature of CH3+ (T = 660 ± 80 K) in d203-506 to be derived. A beam-averaged column density of CH3+ in this protoplanetary disk is also estimated.