  We investigate the nature and classification of PMNJ1603-4904, a bright radio
source close to the Galactic plane, which is associated with one of the
brightest hard-spectrum gamma-ray sources detected by Fermi/LAT. It has
previously been classified as a low-peaked BL Lac object based on its broadband
emission and the absence of optical emission lines. Optical measurements,
however, suffer strongly from extinction and the absence of pronounced
short-time gamma-ray variability over years of monitoring is unusual for a
blazar. We are combining new and archival multiwavelength data in order to
reconsider the classification and nature of this unusual gamma-ray source. For
the first time, we study the radio morphology at 8.4GHz and 22.3GHz, and its
spectral properties on milliarcsecond (mas) scales, based on VLBI observations
from the TANAMI program. We combine the resulting images with multiwavelength
data in the radio, IR, optical/UV, X-ray, and gamma-ray regimes. PMNJ1603-4904
shows a symmetric brightness distribution at 8.4GHz on mas-scales, with the
brightest, and most compact component in the center of the emission region. The
morphology is reminiscent of a Compact Symmetric Object (CSO). Such objects
have been predicted to produce gamma-ray emission but have not been detected as
a class by Fermi/LAT so far. Sparse (u, v)-coverage at 22.3GHz prevents an
unambiguous modeling of the source morphology. IR measurements reveal an excess
in the spectral energy distribution (SED), which can be modeled with a
blackbody with a temperature of about 1600K, and which is usually not present
in blazar SEDs. The VLBI data and the shape of the SED challenge the current
blazar classification. PMNJ1603-4904 seems to be either a highly peculiar BL
Lac object or a misaligned jet source. In the latter case, the intriguing VLBI
structure opens room for a possible classification as a gamma-ray bright CSO.
