  The H.E.S.S. Galactic Plane Survey (HGPS) revealed 78 TeV sources among which
47 are not clearly associated with a known object. We present a multiwavelength
approach to constrain the origin of the emission from unidentified HGPS
sources. We present a generic pipeline that explores a large database of
multiwavelength archival data toward any region in the Galactic plane. Along
with a visual inspection of the retrieved multiwavelength observations to
search for faint and uncataloged counterparts, we derive a radio spectral index
that helps disentangle thermal from nonthermal emission and a mean magnetic
field through X-ray and TeV data in case of a leptonic scenario. We also search
for a spectral connection between the GeV and the TeV regimes with the
Fermi-LAT cataloged sources that may be associated with the unidentified HGPS
source. We complete the association procedure with catalogs of known objects
and with the source catalogs from instruments whose data are retrieved. The
method is applied on two unidentified sources, namely HESS J1427$-$608 and HESS
J1458$-$608, for which the multiwavelength constraints favor the pulsar wind
nebula (PWN) scenario. We model their broadband nonthermal spectra in a
leptonic scenario with a magnetic field $B \lesssim 10$ $\mu$G, which is
consistent with that obtained from ancient PWNe. We place both sources within
the context of the TeV PWN population to estimate the spin-down power and the
characteristic age of the putative pulsar. We also shed light on two possibly
significant $\gamma$-ray excesses in the HGPS: the first is located in the
south of the unidentified source HESS J1632$-$478 and the second is spatially
coincident with the synchrotron-emitting supernova remnant G28.6$-$0.1. The
multiwavelength counterparts found toward both $\gamma$-ray excesses make these
promising candidates for being new very-high energy $\gamma$-ray sources.
